Created starter files for the project.

venv/Lib/site-packages/pip/_internal/resolution/base.py

@@ -0,0 +1,20 @@
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+if MYPY_CHECK_RUNNING:
+    from typing import Callable, List
+    from pip._internal.req.req_install import InstallRequirement
+    from pip._internal.req.req_set import RequirementSet
+
+    InstallRequirementProvider = Callable[
+        [str, InstallRequirement], InstallRequirement
+    ]
+
+
+class BaseResolver(object):
+    def resolve(self, root_reqs, check_supported_wheels):
+        # type: (List[InstallRequirement], bool) -> RequirementSet
+        raise NotImplementedError()
+
+    def get_installation_order(self, req_set):
+        # type: (RequirementSet) -> List[InstallRequirement]
+        raise NotImplementedError()

venv/Lib/site-packages/pip/_internal/resolution/legacy/resolver.py

@@ -0,0 +1,485 @@
+"""Dependency Resolution
+
+The dependency resolution in pip is performed as follows:
+
+for top-level requirements:
+    a. only one spec allowed per project, regardless of conflicts or not.
+       otherwise a "double requirement" exception is raised
+    b. they override sub-dependency requirements.
+for sub-dependencies
+    a. "first found, wins" (where the order is breadth first)
+"""
+
+# The following comment should be removed at some point in the future.
+# mypy: strict-optional=False
+# mypy: disallow-untyped-defs=False
+
+import logging
+import sys
+from collections import defaultdict
+from itertools import chain
+
+from pip._vendor.packaging import specifiers
+
+from pip._internal.exceptions import (
+    BestVersionAlreadyInstalled,
+    DistributionNotFound,
+    HashError,
+    HashErrors,
+    UnsupportedPythonVersion,
+)
+from pip._internal.req.req_install import check_invalid_constraint_type
+from pip._internal.req.req_set import RequirementSet
+from pip._internal.resolution.base import BaseResolver
+from pip._internal.utils.compatibility_tags import get_supported
+from pip._internal.utils.logging import indent_log
+from pip._internal.utils.misc import dist_in_usersite, normalize_version_info
+from pip._internal.utils.packaging import (
+    check_requires_python,
+    get_requires_python,
+)
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+if MYPY_CHECK_RUNNING:
+    from typing import DefaultDict, List, Optional, Set, Tuple
+    from pip._vendor import pkg_resources
+
+    from pip._internal.cache import WheelCache
+    from pip._internal.distributions import AbstractDistribution
+    from pip._internal.index.package_finder import PackageFinder
+    from pip._internal.models.link import Link
+    from pip._internal.operations.prepare import RequirementPreparer
+    from pip._internal.req.req_install import InstallRequirement
+    from pip._internal.resolution.base import InstallRequirementProvider
+
+    DiscoveredDependencies = DefaultDict[str, List[InstallRequirement]]
+
+logger = logging.getLogger(__name__)
+
+
+def _check_dist_requires_python(
+    dist,  # type: pkg_resources.Distribution
+    version_info,  # type: Tuple[int, int, int]
+    ignore_requires_python=False,  # type: bool
+):
+    # type: (...) -> None
+    """
+    Check whether the given Python version is compatible with a distribution's
+    "Requires-Python" value.
+
+    :param version_info: A 3-tuple of ints representing the Python
+        major-minor-micro version to check.
+    :param ignore_requires_python: Whether to ignore the "Requires-Python"
+        value if the given Python version isn't compatible.
+
+    :raises UnsupportedPythonVersion: When the given Python version isn't
+        compatible.
+    """
+    requires_python = get_requires_python(dist)
+    try:
+        is_compatible = check_requires_python(
+            requires_python, version_info=version_info,
+        )
+    except specifiers.InvalidSpecifier as exc:
+        logger.warning(
+            "Package %r has an invalid Requires-Python: %s",
+            dist.project_name, exc,
+        )
+        return
+
+    if is_compatible:
+        return
+
+    version = '.'.join(map(str, version_info))
+    if ignore_requires_python:
+        logger.debug(
+            'Ignoring failed Requires-Python check for package %r: '
+            '%s not in %r',
+            dist.project_name, version, requires_python,
+        )
+        return
+
+    raise UnsupportedPythonVersion(
+        'Package {!r} requires a different Python: {} not in {!r}'.format(
+            dist.project_name, version, requires_python,
+        ))
+
+
+class Resolver(BaseResolver):
+    """Resolves which packages need to be installed/uninstalled to perform \
+    the requested operation without breaking the requirements of any package.
+    """
+
+    _allowed_strategies = {"eager", "only-if-needed", "to-satisfy-only"}
+
+    def __init__(
+        self,
+        preparer,  # type: RequirementPreparer
+        finder,  # type: PackageFinder
+        wheel_cache,  # type: Optional[WheelCache]
+        make_install_req,  # type: InstallRequirementProvider
+        use_user_site,  # type: bool
+        ignore_dependencies,  # type: bool
+        ignore_installed,  # type: bool
+        ignore_requires_python,  # type: bool
+        force_reinstall,  # type: bool
+        upgrade_strategy,  # type: str
+        py_version_info=None,  # type: Optional[Tuple[int, ...]]
+    ):
+        # type: (...) -> None
+        super(Resolver, self).__init__()
+        assert upgrade_strategy in self._allowed_strategies
+
+        if py_version_info is None:
+            py_version_info = sys.version_info[:3]
+        else:
+            py_version_info = normalize_version_info(py_version_info)
+
+        self._py_version_info = py_version_info
+
+        self.preparer = preparer
+        self.finder = finder
+        self.wheel_cache = wheel_cache
+
+        self.upgrade_strategy = upgrade_strategy
+        self.force_reinstall = force_reinstall
+        self.ignore_dependencies = ignore_dependencies
+        self.ignore_installed = ignore_installed
+        self.ignore_requires_python = ignore_requires_python
+        self.use_user_site = use_user_site
+        self._make_install_req = make_install_req
+
+        self._discovered_dependencies = \
+            defaultdict(list)  # type: DiscoveredDependencies
+
+    def resolve(self, root_reqs, check_supported_wheels):
+        # type: (List[InstallRequirement], bool) -> RequirementSet
+        """Resolve what operations need to be done
+
+        As a side-effect of this method, the packages (and their dependencies)
+        are downloaded, unpacked and prepared for installation. This
+        preparation is done by ``pip.operations.prepare``.
+
+        Once PyPI has static dependency metadata available, it would be
+        possible to move the preparation to become a step separated from
+        dependency resolution.
+        """
+        requirement_set = RequirementSet(
+            check_supported_wheels=check_supported_wheels
+        )
+        for req in root_reqs:
+            if req.constraint:
+                check_invalid_constraint_type(req)
+            requirement_set.add_requirement(req)
+
+        # Actually prepare the files, and collect any exceptions. Most hash
+        # exceptions cannot be checked ahead of time, because
+        # _populate_link() needs to be called before we can make decisions
+        # based on link type.
+        discovered_reqs = []  # type: List[InstallRequirement]
+        hash_errors = HashErrors()
+        for req in chain(requirement_set.all_requirements, discovered_reqs):
+            try:
+                discovered_reqs.extend(self._resolve_one(requirement_set, req))
+            except HashError as exc:
+                exc.req = req
+                hash_errors.append(exc)
+
+        if hash_errors:
+            raise hash_errors
+
+        return requirement_set
+
+    def _is_upgrade_allowed(self, req):
+        # type: (InstallRequirement) -> bool
+        if self.upgrade_strategy == "to-satisfy-only":
+            return False
+        elif self.upgrade_strategy == "eager":
+            return True
+        else:
+            assert self.upgrade_strategy == "only-if-needed"
+            return req.user_supplied or req.constraint
+
+    def _set_req_to_reinstall(self, req):
+        # type: (InstallRequirement) -> None
+        """
+        Set a requirement to be installed.
+        """
+        # Don't uninstall the conflict if doing a user install and the
+        # conflict is not a user install.
+        if not self.use_user_site or dist_in_usersite(req.satisfied_by):
+            req.should_reinstall = True
+        req.satisfied_by = None
+
+    def _check_skip_installed(self, req_to_install):
+        # type: (InstallRequirement) -> Optional[str]
+        """Check if req_to_install should be skipped.
+
+        This will check if the req is installed, and whether we should upgrade
+        or reinstall it, taking into account all the relevant user options.
+
+        After calling this req_to_install will only have satisfied_by set to
+        None if the req_to_install is to be upgraded/reinstalled etc. Any
+        other value will be a dist recording the current thing installed that
+        satisfies the requirement.
+
+        Note that for vcs urls and the like we can't assess skipping in this
+        routine - we simply identify that we need to pull the thing down,
+        then later on it is pulled down and introspected to assess upgrade/
+        reinstalls etc.
+
+        :return: A text reason for why it was skipped, or None.
+        """
+        if self.ignore_installed:
+            return None
+
+        req_to_install.check_if_exists(self.use_user_site)
+        if not req_to_install.satisfied_by:
+            return None
+
+        if self.force_reinstall:
+            self._set_req_to_reinstall(req_to_install)
+            return None
+
+        if not self._is_upgrade_allowed(req_to_install):
+            if self.upgrade_strategy == "only-if-needed":
+                return 'already satisfied, skipping upgrade'
+            return 'already satisfied'
+
+        # Check for the possibility of an upgrade.  For link-based
+        # requirements we have to pull the tree down and inspect to assess
+        # the version #, so it's handled way down.
+        if not req_to_install.link:
+            try:
+                self.finder.find_requirement(req_to_install, upgrade=True)
+            except BestVersionAlreadyInstalled:
+                # Then the best version is installed.
+                return 'already up-to-date'
+            except DistributionNotFound:
+                # No distribution found, so we squash the error.  It will
+                # be raised later when we re-try later to do the install.
+                # Why don't we just raise here?
+                pass
+
+        self._set_req_to_reinstall(req_to_install)
+        return None
+
+    def _find_requirement_link(self, req):
+        # type: (InstallRequirement) -> Optional[Link]
+        upgrade = self._is_upgrade_allowed(req)
+        best_candidate = self.finder.find_requirement(req, upgrade)
+        if not best_candidate:
+            return None
+
+        # Log a warning per PEP 592 if necessary before returning.
+        link = best_candidate.link
+        if link.is_yanked:
+            reason = link.yanked_reason or '<none given>'
+            msg = (
+                # Mark this as a unicode string to prevent
+                # "UnicodeEncodeError: 'ascii' codec can't encode character"
+                # in Python 2 when the reason contains non-ascii characters.
+                u'The candidate selected for download or install is a '
+                'yanked version: {candidate}\n'
+                'Reason for being yanked: {reason}'
+            ).format(candidate=best_candidate, reason=reason)
+            logger.warning(msg)
+
+        return link
+
+    def _populate_link(self, req):
+        # type: (InstallRequirement) -> None
+        """Ensure that if a link can be found for this, that it is found.
+
+        Note that req.link may still be None - if the requirement is already
+        installed and not needed to be upgraded based on the return value of
+        _is_upgrade_allowed().
+
+        If preparer.require_hashes is True, don't use the wheel cache, because
+        cached wheels, always built locally, have different hashes than the
+        files downloaded from the index server and thus throw false hash
+        mismatches. Furthermore, cached wheels at present have undeterministic
+        contents due to file modification times.
+        """
+        if req.link is None:
+            req.link = self._find_requirement_link(req)
+
+        if self.wheel_cache is None or self.preparer.require_hashes:
+            return
+        cache_entry = self.wheel_cache.get_cache_entry(
+            link=req.link,
+            package_name=req.name,
+            supported_tags=get_supported(),
+        )
+        if cache_entry is not None:
+            logger.debug('Using cached wheel link: %s', cache_entry.link)
+            if req.link is req.original_link and cache_entry.persistent:
+                req.original_link_is_in_wheel_cache = True
+            req.link = cache_entry.link
+
+    def _get_abstract_dist_for(self, req):
+        # type: (InstallRequirement) -> AbstractDistribution
+        """Takes a InstallRequirement and returns a single AbstractDist \
+        representing a prepared variant of the same.
+        """
+        if req.editable:
+            return self.preparer.prepare_editable_requirement(req)
+
+        # satisfied_by is only evaluated by calling _check_skip_installed,
+        # so it must be None here.
+        assert req.satisfied_by is None
+        skip_reason = self._check_skip_installed(req)
+
+        if req.satisfied_by:
+            return self.preparer.prepare_installed_requirement(
+                req, skip_reason
+            )
+
+        # We eagerly populate the link, since that's our "legacy" behavior.
+        self._populate_link(req)
+        abstract_dist = self.preparer.prepare_linked_requirement(req)
+
+        # NOTE
+        # The following portion is for determining if a certain package is
+        # going to be re-installed/upgraded or not and reporting to the user.
+        # This should probably get cleaned up in a future refactor.
+
+        # req.req is only avail after unpack for URL
+        # pkgs repeat check_if_exists to uninstall-on-upgrade
+        # (#14)
+        if not self.ignore_installed:
+            req.check_if_exists(self.use_user_site)
+
+        if req.satisfied_by:
+            should_modify = (
+                self.upgrade_strategy != "to-satisfy-only" or
+                self.force_reinstall or
+                self.ignore_installed or
+                req.link.scheme == 'file'
+            )
+            if should_modify:
+                self._set_req_to_reinstall(req)
+            else:
+                logger.info(
+                    'Requirement already satisfied (use --upgrade to upgrade):'
+                    ' %s', req,
+                )
+
+        return abstract_dist
+
+    def _resolve_one(
+        self,
+        requirement_set,  # type: RequirementSet
+        req_to_install,  # type: InstallRequirement
+    ):
+        # type: (...) -> List[InstallRequirement]
+        """Prepare a single requirements file.
+
+        :return: A list of additional InstallRequirements to also install.
+        """
+        # Tell user what we are doing for this requirement:
+        # obtain (editable), skipping, processing (local url), collecting
+        # (remote url or package name)
+        if req_to_install.constraint or req_to_install.prepared:
+            return []
+
+        req_to_install.prepared = True
+
+        abstract_dist = self._get_abstract_dist_for(req_to_install)
+
+        # Parse and return dependencies
+        dist = abstract_dist.get_pkg_resources_distribution()
+        # This will raise UnsupportedPythonVersion if the given Python
+        # version isn't compatible with the distribution's Requires-Python.
+        _check_dist_requires_python(
+            dist, version_info=self._py_version_info,
+            ignore_requires_python=self.ignore_requires_python,
+        )
+
+        more_reqs = []  # type: List[InstallRequirement]
+
+        def add_req(subreq, extras_requested):
+            sub_install_req = self._make_install_req(
+                str(subreq),
+                req_to_install,
+            )
+            parent_req_name = req_to_install.name
+            to_scan_again, add_to_parent = requirement_set.add_requirement(
+                sub_install_req,
+                parent_req_name=parent_req_name,
+                extras_requested=extras_requested,
+            )
+            if parent_req_name and add_to_parent:
+                self._discovered_dependencies[parent_req_name].append(
+                    add_to_parent
+                )
+            more_reqs.extend(to_scan_again)
+
+        with indent_log():
+            # We add req_to_install before its dependencies, so that we
+            # can refer to it when adding dependencies.
+            if not requirement_set.has_requirement(req_to_install.name):
+                # 'unnamed' requirements will get added here
+                # 'unnamed' requirements can only come from being directly
+                # provided by the user.
+                assert req_to_install.user_supplied
+                requirement_set.add_requirement(
+                    req_to_install, parent_req_name=None,
+                )
+
+            if not self.ignore_dependencies:
+                if req_to_install.extras:
+                    logger.debug(
+                        "Installing extra requirements: %r",
+                        ','.join(req_to_install.extras),
+                    )
+                missing_requested = sorted(
+                    set(req_to_install.extras) - set(dist.extras)
+                )
+                for missing in missing_requested:
+                    logger.warning(
+                        "%s does not provide the extra '%s'",
+                        dist, missing
+                    )
+
+                available_requested = sorted(
+                    set(dist.extras) & set(req_to_install.extras)
+                )
+                for subreq in dist.requires(available_requested):
+                    add_req(subreq, extras_requested=available_requested)
+
+            if not req_to_install.editable and not req_to_install.satisfied_by:
+                # XXX: --no-install leads this to report 'Successfully
+                # downloaded' for only non-editable reqs, even though we took
+                # action on them.
+                req_to_install.successfully_downloaded = True
+
+        return more_reqs
+
+    def get_installation_order(self, req_set):
+        # type: (RequirementSet) -> List[InstallRequirement]
+        """Create the installation order.
+
+        The installation order is topological - requirements are installed
+        before the requiring thing. We break cycles at an arbitrary point,
+        and make no other guarantees.
+        """
+        # The current implementation, which we may change at any point
+        # installs the user specified things in the order given, except when
+        # dependencies must come earlier to achieve topological order.
+        order = []
+        ordered_reqs = set()  # type: Set[InstallRequirement]
+
+        def schedule(req):
+            if req.satisfied_by or req in ordered_reqs:
+                return
+            if req.constraint:
+                return
+            ordered_reqs.add(req)
+            for dep in self._discovered_dependencies[req.name]:
+                schedule(dep)
+            order.append(req)
+
+        for install_req in req_set.requirements.values():
+            schedule(install_req)
+        return order

venv/Lib/site-packages/pip/_internal/resolution/resolvelib/base.py

@@ -0,0 +1,82 @@
+from pip._vendor.packaging.utils import canonicalize_name
+
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+if MYPY_CHECK_RUNNING:
+    from typing import FrozenSet, Iterable, Optional, Tuple
+
+    from pip._vendor.packaging.version import _BaseVersion
+
+    from pip._internal.models.link import Link
+    from pip._internal.req.req_install import InstallRequirement
+
+    CandidateLookup = Tuple[
+        Optional["Candidate"],
+        Optional[InstallRequirement],
+    ]
+
+
+def format_name(project, extras):
+    # type: (str, FrozenSet[str]) -> str
+    if not extras:
+        return project
+    canonical_extras = sorted(canonicalize_name(e) for e in extras)
+    return "{}[{}]".format(project, ",".join(canonical_extras))
+
+
+class Requirement(object):
+    @property
+    def name(self):
+        # type: () -> str
+        raise NotImplementedError("Subclass should override")
+
+    def is_satisfied_by(self, candidate):
+        # type: (Candidate) -> bool
+        return False
+
+    def get_candidate_lookup(self):
+        # type: () -> CandidateLookup
+        raise NotImplementedError("Subclass should override")
+
+    def format_for_error(self):
+        # type: () -> str
+        raise NotImplementedError("Subclass should override")
+
+
+class Candidate(object):
+    @property
+    def name(self):
+        # type: () -> str
+        raise NotImplementedError("Override in subclass")
+
+    @property
+    def version(self):
+        # type: () -> _BaseVersion
+        raise NotImplementedError("Override in subclass")
+
+    @property
+    def is_installed(self):
+        # type: () -> bool
+        raise NotImplementedError("Override in subclass")
+
+    @property
+    def is_editable(self):
+        # type: () -> bool
+        raise NotImplementedError("Override in subclass")
+
+    @property
+    def source_link(self):
+        # type: () -> Optional[Link]
+        raise NotImplementedError("Override in subclass")
+
+    def iter_dependencies(self, with_requires):
+        # type: (bool) -> Iterable[Optional[Requirement]]
+        raise NotImplementedError("Override in subclass")
+
+    def get_install_requirement(self):
+        # type: () -> Optional[InstallRequirement]
+        raise NotImplementedError("Override in subclass")
+
+    def format_for_error(self):
+        # type: () -> str
+        raise NotImplementedError("Subclass should override")

venv/Lib/site-packages/pip/_internal/resolution/resolvelib/candidates.py

@@ -0,0 +1,600 @@
+import logging
+import sys
+
+from pip._vendor.contextlib2 import suppress
+from pip._vendor.packaging.specifiers import InvalidSpecifier, SpecifierSet
+from pip._vendor.packaging.utils import canonicalize_name
+from pip._vendor.packaging.version import Version
+
+from pip._internal.exceptions import HashError, MetadataInconsistent
+from pip._internal.network.lazy_wheel import (
+    HTTPRangeRequestUnsupported,
+    dist_from_wheel_url,
+)
+from pip._internal.req.constructors import (
+    install_req_from_editable,
+    install_req_from_line,
+)
+from pip._internal.req.req_install import InstallRequirement
+from pip._internal.utils.logging import indent_log
+from pip._internal.utils.misc import dist_is_editable, normalize_version_info
+from pip._internal.utils.packaging import get_requires_python
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+from .base import Candidate, format_name
+
+if MYPY_CHECK_RUNNING:
+    from typing import Any, FrozenSet, Iterable, Optional, Tuple, Union
+
+    from pip._vendor.packaging.version import _BaseVersion
+    from pip._vendor.pkg_resources import Distribution
+
+    from pip._internal.distributions import AbstractDistribution
+    from pip._internal.models.link import Link
+
+    from .base import Requirement
+    from .factory import Factory
+
+    BaseCandidate = Union[
+        "AlreadyInstalledCandidate",
+        "EditableCandidate",
+        "LinkCandidate",
+    ]
+
+
+logger = logging.getLogger(__name__)
+
+
+def make_install_req_from_link(link, template):
+    # type: (Link, InstallRequirement) -> InstallRequirement
+    assert not template.editable, "template is editable"
+    if template.req:
+        line = str(template.req)
+    else:
+        line = link.url
+    ireq = install_req_from_line(
+        line,
+        user_supplied=template.user_supplied,
+        comes_from=template.comes_from,
+        use_pep517=template.use_pep517,
+        isolated=template.isolated,
+        constraint=template.constraint,
+        options=dict(
+            install_options=template.install_options,
+            global_options=template.global_options,
+            hashes=template.hash_options
+        ),
+    )
+    ireq.original_link = template.original_link
+    ireq.link = link
+    return ireq
+
+
+def make_install_req_from_editable(link, template):
+    # type: (Link, InstallRequirement) -> InstallRequirement
+    assert template.editable, "template not editable"
+    return install_req_from_editable(
+        link.url,
+        user_supplied=template.user_supplied,
+        comes_from=template.comes_from,
+        use_pep517=template.use_pep517,
+        isolated=template.isolated,
+        constraint=template.constraint,
+        options=dict(
+            install_options=template.install_options,
+            global_options=template.global_options,
+            hashes=template.hash_options
+        ),
+    )
+
+
+def make_install_req_from_dist(dist, template):
+    # type: (Distribution, InstallRequirement) -> InstallRequirement
+    project_name = canonicalize_name(dist.project_name)
+    if template.req:
+        line = str(template.req)
+    elif template.link:
+        line = "{} @ {}".format(project_name, template.link.url)
+    else:
+        line = "{}=={}".format(project_name, dist.parsed_version)
+    ireq = install_req_from_line(
+        line,
+        user_supplied=template.user_supplied,
+        comes_from=template.comes_from,
+        use_pep517=template.use_pep517,
+        isolated=template.isolated,
+        constraint=template.constraint,
+        options=dict(
+            install_options=template.install_options,
+            global_options=template.global_options,
+            hashes=template.hash_options
+        ),
+    )
+    ireq.satisfied_by = dist
+    return ireq
+
+
+class _InstallRequirementBackedCandidate(Candidate):
+    """A candidate backed by an ``InstallRequirement``.
+
+    This represents a package request with the target not being already
+    in the environment, and needs to be fetched and installed. The backing
+    ``InstallRequirement`` is responsible for most of the leg work; this
+    class exposes appropriate information to the resolver.
+
+    :param link: The link passed to the ``InstallRequirement``. The backing
+        ``InstallRequirement`` will use this link to fetch the distribution.
+    :param source_link: The link this candidate "originates" from. This is
+        different from ``link`` when the link is found in the wheel cache.
+        ``link`` would point to the wheel cache, while this points to the
+        found remote link (e.g. from pypi.org).
+    """
+    is_installed = False
+
+    def __init__(
+        self,
+        link,          # type: Link
+        source_link,   # type: Link
+        ireq,          # type: InstallRequirement
+        factory,       # type: Factory
+        name=None,     # type: Optional[str]
+        version=None,  # type: Optional[_BaseVersion]
+    ):
+        # type: (...) -> None
+        self._link = link
+        self._source_link = source_link
+        self._factory = factory
+        self._ireq = ireq
+        self._name = name
+        self._version = version
+        self._dist = None  # type: Optional[Distribution]
+        self._prepared = False
+
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}({link!r})".format(
+            class_name=self.__class__.__name__,
+            link=str(self._link),
+        )
+
+    def __hash__(self):
+        # type: () -> int
+        return hash((self.__class__, self._link))
+
+    def __eq__(self, other):
+        # type: (Any) -> bool
+        if isinstance(other, self.__class__):
+            return self._link == other._link
+        return False
+
+    # Needed for Python 2, which does not implement this by default
+    def __ne__(self, other):
+        # type: (Any) -> bool
+        return not self.__eq__(other)
+
+    @property
+    def source_link(self):
+        # type: () -> Optional[Link]
+        return self._source_link
+
+    @property
+    def name(self):
+        # type: () -> str
+        """The normalised name of the project the candidate refers to"""
+        if self._name is None:
+            self._name = canonicalize_name(self.dist.project_name)
+        return self._name
+
+    @property
+    def version(self):
+        # type: () -> _BaseVersion
+        if self._version is None:
+            self._version = self.dist.parsed_version
+        return self._version
+
+    def format_for_error(self):
+        # type: () -> str
+        return "{} {} (from {})".format(
+            self.name,
+            self.version,
+            self._link.file_path if self._link.is_file else self._link
+        )
+
+    def _prepare_abstract_distribution(self):
+        # type: () -> AbstractDistribution
+        raise NotImplementedError("Override in subclass")
+
+    def _check_metadata_consistency(self):
+        # type: () -> None
+        """Check for consistency of project name and version of dist."""
+        # TODO: (Longer term) Rather than abort, reject this candidate
+        #       and backtrack. This would need resolvelib support.
+        dist = self._dist  # type: Distribution
+        name = canonicalize_name(dist.project_name)
+        if self._name is not None and self._name != name:
+            raise MetadataInconsistent(self._ireq, "name", dist.project_name)
+        version = dist.parsed_version
+        if self._version is not None and self._version != version:
+            raise MetadataInconsistent(self._ireq, "version", dist.version)
+
+    def _prepare(self):
+        # type: () -> None
+        if self._prepared:
+            return
+        try:
+            abstract_dist = self._prepare_abstract_distribution()
+        except HashError as e:
+            e.req = self._ireq
+            raise
+
+        self._dist = abstract_dist.get_pkg_resources_distribution()
+        assert self._dist is not None, "Distribution already installed"
+        self._check_metadata_consistency()
+        self._prepared = True
+
+    def _fetch_metadata(self):
+        # type: () -> None
+        """Fetch metadata, using lazy wheel if possible."""
+        preparer = self._factory.preparer
+        use_lazy_wheel = self._factory.use_lazy_wheel
+        remote_wheel = self._link.is_wheel and not self._link.is_file
+        if use_lazy_wheel and remote_wheel and not preparer.require_hashes:
+            assert self._name is not None
+            logger.info('Collecting %s', self._ireq.req or self._ireq)
+            # If HTTPRangeRequestUnsupported is raised, fallback silently.
+            with indent_log(), suppress(HTTPRangeRequestUnsupported):
+                logger.info(
+                    'Obtaining dependency information from %s %s',
+                    self._name, self._version,
+                )
+                url = self._link.url.split('#', 1)[0]
+                session = preparer.downloader._session
+                self._dist = dist_from_wheel_url(self._name, url, session)
+                self._check_metadata_consistency()
+        if self._dist is None:
+            self._prepare()
+
+    @property
+    def dist(self):
+        # type: () -> Distribution
+        if self._dist is None:
+            self._fetch_metadata()
+        return self._dist
+
+    def _get_requires_python_specifier(self):
+        # type: () -> Optional[SpecifierSet]
+        requires_python = get_requires_python(self.dist)
+        if requires_python is None:
+            return None
+        try:
+            spec = SpecifierSet(requires_python)
+        except InvalidSpecifier as e:
+            logger.warning(
+                "Package %r has an invalid Requires-Python: %s", self.name, e,
+            )
+            return None
+        return spec
+
+    def iter_dependencies(self, with_requires):
+        # type: (bool) -> Iterable[Optional[Requirement]]
+        if not with_requires:
+            return
+        for r in self.dist.requires():
+            yield self._factory.make_requirement_from_spec(str(r), self._ireq)
+        python_dep = self._factory.make_requires_python_requirement(
+            self._get_requires_python_specifier(),
+        )
+        if python_dep:
+            yield python_dep
+
+    def get_install_requirement(self):
+        # type: () -> Optional[InstallRequirement]
+        self._prepare()
+        return self._ireq
+
+
+class LinkCandidate(_InstallRequirementBackedCandidate):
+    is_editable = False
+
+    def __init__(
+        self,
+        link,          # type: Link
+        template,        # type: InstallRequirement
+        factory,       # type: Factory
+        name=None,     # type: Optional[str]
+        version=None,  # type: Optional[_BaseVersion]
+    ):
+        # type: (...) -> None
+        source_link = link
+        cache_entry = factory.get_wheel_cache_entry(link, name)
+        if cache_entry is not None:
+            logger.debug("Using cached wheel link: %s", cache_entry.link)
+            link = cache_entry.link
+        ireq = make_install_req_from_link(link, template)
+
+        if (cache_entry is not None and
+                cache_entry.persistent and
+                template.link is template.original_link):
+            ireq.original_link_is_in_wheel_cache = True
+
+        super(LinkCandidate, self).__init__(
+            link=link,
+            source_link=source_link,
+            ireq=ireq,
+            factory=factory,
+            name=name,
+            version=version,
+        )
+
+    def _prepare_abstract_distribution(self):
+        # type: () -> AbstractDistribution
+        return self._factory.preparer.prepare_linked_requirement(
+            self._ireq, parallel_builds=True,
+        )
+
+
+class EditableCandidate(_InstallRequirementBackedCandidate):
+    is_editable = True
+
+    def __init__(
+        self,
+        link,          # type: Link
+        template,        # type: InstallRequirement
+        factory,       # type: Factory
+        name=None,     # type: Optional[str]
+        version=None,  # type: Optional[_BaseVersion]
+    ):
+        # type: (...) -> None
+        super(EditableCandidate, self).__init__(
+            link=link,
+            source_link=link,
+            ireq=make_install_req_from_editable(link, template),
+            factory=factory,
+            name=name,
+            version=version,
+        )
+
+    def _prepare_abstract_distribution(self):
+        # type: () -> AbstractDistribution
+        return self._factory.preparer.prepare_editable_requirement(self._ireq)
+
+
+class AlreadyInstalledCandidate(Candidate):
+    is_installed = True
+    source_link = None
+
+    def __init__(
+        self,
+        dist,  # type: Distribution
+        template,  # type: InstallRequirement
+        factory,  # type: Factory
+    ):
+        # type: (...) -> None
+        self.dist = dist
+        self._ireq = make_install_req_from_dist(dist, template)
+        self._factory = factory
+
+        # This is just logging some messages, so we can do it eagerly.
+        # The returned dist would be exactly the same as self.dist because we
+        # set satisfied_by in make_install_req_from_dist.
+        # TODO: Supply reason based on force_reinstall and upgrade_strategy.
+        skip_reason = "already satisfied"
+        factory.preparer.prepare_installed_requirement(self._ireq, skip_reason)
+
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}({distribution!r})".format(
+            class_name=self.__class__.__name__,
+            distribution=self.dist,
+        )
+
+    def __hash__(self):
+        # type: () -> int
+        return hash((self.__class__, self.name, self.version))
+
+    def __eq__(self, other):
+        # type: (Any) -> bool
+        if isinstance(other, self.__class__):
+            return self.name == other.name and self.version == other.version
+        return False
+
+    # Needed for Python 2, which does not implement this by default
+    def __ne__(self, other):
+        # type: (Any) -> bool
+        return not self.__eq__(other)
+
+    @property
+    def name(self):
+        # type: () -> str
+        return canonicalize_name(self.dist.project_name)
+
+    @property
+    def version(self):
+        # type: () -> _BaseVersion
+        return self.dist.parsed_version
+
+    @property
+    def is_editable(self):
+        # type: () -> bool
+        return dist_is_editable(self.dist)
+
+    def format_for_error(self):
+        # type: () -> str
+        return "{} {} (Installed)".format(self.name, self.version)
+
+    def iter_dependencies(self, with_requires):
+        # type: (bool) -> Iterable[Optional[Requirement]]
+        if not with_requires:
+            return
+        for r in self.dist.requires():
+            yield self._factory.make_requirement_from_spec(str(r), self._ireq)
+
+    def get_install_requirement(self):
+        # type: () -> Optional[InstallRequirement]
+        return None
+
+
+class ExtrasCandidate(Candidate):
+    """A candidate that has 'extras', indicating additional dependencies.
+
+    Requirements can be for a project with dependencies, something like
+    foo[extra].  The extras don't affect the project/version being installed
+    directly, but indicate that we need additional dependencies. We model that
+    by having an artificial ExtrasCandidate that wraps the "base" candidate.
+
+    The ExtrasCandidate differs from the base in the following ways:
+
+    1. It has a unique name, of the form foo[extra]. This causes the resolver
+       to treat it as a separate node in the dependency graph.
+    2. When we're getting the candidate's dependencies,
+       a) We specify that we want the extra dependencies as well.
+       b) We add a dependency on the base candidate.
+          See below for why this is needed.
+    3. We return None for the underlying InstallRequirement, as the base
+       candidate will provide it, and we don't want to end up with duplicates.
+
+    The dependency on the base candidate is needed so that the resolver can't
+    decide that it should recommend foo[extra1] version 1.0 and foo[extra2]
+    version 2.0. Having those candidates depend on foo=1.0 and foo=2.0
+    respectively forces the resolver to recognise that this is a conflict.
+    """
+    def __init__(
+        self,
+        base,  # type: BaseCandidate
+        extras,  # type: FrozenSet[str]
+    ):
+        # type: (...) -> None
+        self.base = base
+        self.extras = extras
+
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}(base={base!r}, extras={extras!r})".format(
+            class_name=self.__class__.__name__,
+            base=self.base,
+            extras=self.extras,
+        )
+
+    def __hash__(self):
+        # type: () -> int
+        return hash((self.base, self.extras))
+
+    def __eq__(self, other):
+        # type: (Any) -> bool
+        if isinstance(other, self.__class__):
+            return self.base == other.base and self.extras == other.extras
+        return False
+
+    # Needed for Python 2, which does not implement this by default
+    def __ne__(self, other):
+        # type: (Any) -> bool
+        return not self.__eq__(other)
+
+    @property
+    def name(self):
+        # type: () -> str
+        """The normalised name of the project the candidate refers to"""
+        return format_name(self.base.name, self.extras)
+
+    @property
+    def version(self):
+        # type: () -> _BaseVersion
+        return self.base.version
+
+    def format_for_error(self):
+        # type: () -> str
+        return "{} [{}]".format(
+            self.base.format_for_error(),
+            ", ".join(sorted(self.extras))
+        )
+
+    @property
+    def is_installed(self):
+        # type: () -> bool
+        return self.base.is_installed
+
+    @property
+    def is_editable(self):
+        # type: () -> bool
+        return self.base.is_editable
+
+    @property
+    def source_link(self):
+        # type: () -> Optional[Link]
+        return self.base.source_link
+
+    def iter_dependencies(self, with_requires):
+        # type: (bool) -> Iterable[Optional[Requirement]]
+        factory = self.base._factory
+
+        # Add a dependency on the exact base
+        # (See note 2b in the class docstring)
+        yield factory.make_requirement_from_candidate(self.base)
+        if not with_requires:
+            return
+
+        # The user may have specified extras that the candidate doesn't
+        # support. We ignore any unsupported extras here.
+        valid_extras = self.extras.intersection(self.base.dist.extras)
+        invalid_extras = self.extras.difference(self.base.dist.extras)
+        for extra in sorted(invalid_extras):
+            logger.warning(
+                "%s %s does not provide the extra '%s'",
+                self.base.name,
+                self.version,
+                extra
+            )
+
+        for r in self.base.dist.requires(valid_extras):
+            requirement = factory.make_requirement_from_spec(
+                str(r), self.base._ireq, valid_extras,
+            )
+            if requirement:
+                yield requirement
+
+    def get_install_requirement(self):
+        # type: () -> Optional[InstallRequirement]
+        # We don't return anything here, because we always
+        # depend on the base candidate, and we'll get the
+        # install requirement from that.
+        return None
+
+
+class RequiresPythonCandidate(Candidate):
+    is_installed = False
+    source_link = None
+
+    def __init__(self, py_version_info):
+        # type: (Optional[Tuple[int, ...]]) -> None
+        if py_version_info is not None:
+            version_info = normalize_version_info(py_version_info)
+        else:
+            version_info = sys.version_info[:3]
+        self._version = Version(".".join(str(c) for c in version_info))
+
+    # We don't need to implement __eq__() and __ne__() since there is always
+    # only one RequiresPythonCandidate in a resolution, i.e. the host Python.
+    # The built-in object.__eq__() and object.__ne__() do exactly what we want.
+
+    @property
+    def name(self):
+        # type: () -> str
+        # Avoid conflicting with the PyPI package "Python".
+        return "<Python from Requires-Python>"
+
+    @property
+    def version(self):
+        # type: () -> _BaseVersion
+        return self._version
+
+    def format_for_error(self):
+        # type: () -> str
+        return "Python {}".format(self.version)
+
+    def iter_dependencies(self, with_requires):
+        # type: (bool) -> Iterable[Optional[Requirement]]
+        return ()
+
+    def get_install_requirement(self):
+        # type: () -> Optional[InstallRequirement]
+        return None

venv/Lib/site-packages/pip/_internal/resolution/resolvelib/factory.py

@@ -0,0 +1,459 @@
+import collections
+import logging
+
+from pip._vendor import six
+from pip._vendor.packaging.utils import canonicalize_name
+
+from pip._internal.exceptions import (
+    DistributionNotFound,
+    InstallationError,
+    UnsupportedPythonVersion,
+    UnsupportedWheel,
+)
+from pip._internal.models.wheel import Wheel
+from pip._internal.req.req_install import InstallRequirement
+from pip._internal.utils.compatibility_tags import get_supported
+from pip._internal.utils.hashes import Hashes
+from pip._internal.utils.misc import (
+    dist_in_site_packages,
+    dist_in_usersite,
+    get_installed_distributions,
+)
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+from pip._internal.utils.virtualenv import running_under_virtualenv
+
+from .candidates import (
+    AlreadyInstalledCandidate,
+    EditableCandidate,
+    ExtrasCandidate,
+    LinkCandidate,
+    RequiresPythonCandidate,
+)
+from .requirements import (
+    ExplicitRequirement,
+    RequiresPythonRequirement,
+    SpecifierRequirement,
+)
+
+if MYPY_CHECK_RUNNING:
+    from typing import (
+        FrozenSet,
+        Dict,
+        Iterable,
+        List,
+        Optional,
+        Sequence,
+        Set,
+        Tuple,
+        TypeVar,
+    )
+
+    from pip._vendor.packaging.specifiers import SpecifierSet
+    from pip._vendor.packaging.version import _BaseVersion
+    from pip._vendor.pkg_resources import Distribution
+    from pip._vendor.resolvelib import ResolutionImpossible
+
+    from pip._internal.cache import CacheEntry, WheelCache
+    from pip._internal.index.package_finder import PackageFinder
+    from pip._internal.models.link import Link
+    from pip._internal.operations.prepare import RequirementPreparer
+    from pip._internal.resolution.base import InstallRequirementProvider
+
+    from .base import Candidate, Requirement
+    from .candidates import BaseCandidate
+
+    C = TypeVar("C")
+    Cache = Dict[Link, C]
+    VersionCandidates = Dict[_BaseVersion, Candidate]
+
+
+logger = logging.getLogger(__name__)
+
+
+class Factory(object):
+    def __init__(
+        self,
+        finder,  # type: PackageFinder
+        preparer,  # type: RequirementPreparer
+        make_install_req,  # type: InstallRequirementProvider
+        wheel_cache,  # type: Optional[WheelCache]
+        use_user_site,  # type: bool
+        force_reinstall,  # type: bool
+        ignore_installed,  # type: bool
+        ignore_requires_python,  # type: bool
+        py_version_info=None,  # type: Optional[Tuple[int, ...]]
+        lazy_wheel=False,  # type: bool
+    ):
+        # type: (...) -> None
+        self._finder = finder
+        self.preparer = preparer
+        self._wheel_cache = wheel_cache
+        self._python_candidate = RequiresPythonCandidate(py_version_info)
+        self._make_install_req_from_spec = make_install_req
+        self._use_user_site = use_user_site
+        self._force_reinstall = force_reinstall
+        self._ignore_requires_python = ignore_requires_python
+        self.use_lazy_wheel = lazy_wheel
+
+        self._link_candidate_cache = {}  # type: Cache[LinkCandidate]
+        self._editable_candidate_cache = {}  # type: Cache[EditableCandidate]
+
+        if not ignore_installed:
+            self._installed_dists = {
+                canonicalize_name(dist.project_name): dist
+                for dist in get_installed_distributions()
+            }
+        else:
+            self._installed_dists = {}
+
+    @property
+    def force_reinstall(self):
+        # type: () -> bool
+        return self._force_reinstall
+
+    def _make_candidate_from_dist(
+        self,
+        dist,  # type: Distribution
+        extras,  # type: FrozenSet[str]
+        template,  # type: InstallRequirement
+    ):
+        # type: (...) -> Candidate
+        base = AlreadyInstalledCandidate(dist, template, factory=self)
+        if extras:
+            return ExtrasCandidate(base, extras)
+        return base
+
+    def _make_candidate_from_link(
+        self,
+        link,  # type: Link
+        extras,  # type: FrozenSet[str]
+        template,  # type: InstallRequirement
+        name,  # type: Optional[str]
+        version,  # type: Optional[_BaseVersion]
+    ):
+        # type: (...) -> Candidate
+        # TODO: Check already installed candidate, and use it if the link and
+        # editable flag match.
+        if template.editable:
+            if link not in self._editable_candidate_cache:
+                self._editable_candidate_cache[link] = EditableCandidate(
+                    link, template, factory=self, name=name, version=version,
+                )
+            base = self._editable_candidate_cache[link]  # type: BaseCandidate
+        else:
+            if link not in self._link_candidate_cache:
+                self._link_candidate_cache[link] = LinkCandidate(
+                    link, template, factory=self, name=name, version=version,
+                )
+            base = self._link_candidate_cache[link]
+        if extras:
+            return ExtrasCandidate(base, extras)
+        return base
+
+    def _iter_found_candidates(
+        self,
+        ireqs,  # type: Sequence[InstallRequirement]
+        specifier,  # type: SpecifierSet
+    ):
+        # type: (...) -> Iterable[Candidate]
+        if not ireqs:
+            return ()
+
+        # The InstallRequirement implementation requires us to give it a
+        # "template". Here we just choose the first requirement to represent
+        # all of them.
+        # Hopefully the Project model can correct this mismatch in the future.
+        template = ireqs[0]
+        name = canonicalize_name(template.req.name)
+
+        hashes = Hashes()
+        extras = frozenset()  # type: FrozenSet[str]
+        for ireq in ireqs:
+            specifier &= ireq.req.specifier
+            hashes |= ireq.hashes(trust_internet=False)
+            extras |= frozenset(ireq.extras)
+
+        # We use this to ensure that we only yield a single candidate for
+        # each version (the finder's preferred one for that version). The
+        # requirement needs to return only one candidate per version, so we
+        # implement that logic here so that requirements using this helper
+        # don't all have to do the same thing later.
+        candidates = collections.OrderedDict()  # type: VersionCandidates
+
+        # Get the installed version, if it matches, unless the user
+        # specified `--force-reinstall`, when we want the version from
+        # the index instead.
+        installed_version = None
+        installed_candidate = None
+        if not self._force_reinstall and name in self._installed_dists:
+            installed_dist = self._installed_dists[name]
+            installed_version = installed_dist.parsed_version
+            if specifier.contains(installed_version, prereleases=True):
+                installed_candidate = self._make_candidate_from_dist(
+                    dist=installed_dist,
+                    extras=extras,
+                    template=template,
+                )
+
+        found = self._finder.find_best_candidate(
+            project_name=name,
+            specifier=specifier,
+            hashes=hashes,
+        )
+        for ican in found.iter_applicable():
+            if ican.version == installed_version and installed_candidate:
+                candidate = installed_candidate
+            else:
+                candidate = self._make_candidate_from_link(
+                    link=ican.link,
+                    extras=extras,
+                    template=template,
+                    name=name,
+                    version=ican.version,
+                )
+            candidates[ican.version] = candidate
+
+        # Yield the installed version even if it is not found on the index.
+        if installed_version and installed_candidate:
+            candidates[installed_version] = installed_candidate
+
+        return six.itervalues(candidates)
+
+    def find_candidates(self, requirements, constraint):
+        # type: (Sequence[Requirement], SpecifierSet) -> Iterable[Candidate]
+        explicit_candidates = set()  # type: Set[Candidate]
+        ireqs = []  # type: List[InstallRequirement]
+        for req in requirements:
+            cand, ireq = req.get_candidate_lookup()
+            if cand is not None:
+                explicit_candidates.add(cand)
+            if ireq is not None:
+                ireqs.append(ireq)
+
+        # If none of the requirements want an explicit candidate, we can ask
+        # the finder for candidates.
+        if not explicit_candidates:
+            return self._iter_found_candidates(ireqs, constraint)
+
+        if constraint:
+            name = explicit_candidates.pop().name
+            raise InstallationError(
+                "Could not satisfy constraints for {!r}: installation from "
+                "path or url cannot be constrained to a version".format(name)
+            )
+
+        return (
+            c for c in explicit_candidates
+            if all(req.is_satisfied_by(c) for req in requirements)
+        )
+
+    def make_requirement_from_install_req(self, ireq, requested_extras):
+        # type: (InstallRequirement, Iterable[str]) -> Optional[Requirement]
+        if not ireq.match_markers(requested_extras):
+            logger.info(
+                "Ignoring %s: markers '%s' don't match your environment",
+                ireq.name, ireq.markers,
+            )
+            return None
+        if not ireq.link:
+            return SpecifierRequirement(ireq)
+        if ireq.link.is_wheel:
+            wheel = Wheel(ireq.link.filename)
+            if not wheel.supported(self._finder.target_python.get_tags()):
+                msg = "{} is not a supported wheel on this platform.".format(
+                    wheel.filename,
+                )
+                raise UnsupportedWheel(msg)
+        cand = self._make_candidate_from_link(
+            ireq.link,
+            extras=frozenset(ireq.extras),
+            template=ireq,
+            name=canonicalize_name(ireq.name) if ireq.name else None,
+            version=None,
+        )
+        return self.make_requirement_from_candidate(cand)
+
+    def make_requirement_from_candidate(self, candidate):
+        # type: (Candidate) -> ExplicitRequirement
+        return ExplicitRequirement(candidate)
+
+    def make_requirement_from_spec(
+        self,
+        specifier,  # type: str
+        comes_from,  # type: InstallRequirement
+        requested_extras=(),  # type: Iterable[str]
+    ):
+        # type: (...) -> Optional[Requirement]
+        ireq = self._make_install_req_from_spec(specifier, comes_from)
+        return self.make_requirement_from_install_req(ireq, requested_extras)
+
+    def make_requires_python_requirement(self, specifier):
+        # type: (Optional[SpecifierSet]) -> Optional[Requirement]
+        if self._ignore_requires_python or specifier is None:
+            return None
+        return RequiresPythonRequirement(specifier, self._python_candidate)
+
+    def get_wheel_cache_entry(self, link, name):
+        # type: (Link, Optional[str]) -> Optional[CacheEntry]
+        """Look up the link in the wheel cache.
+
+        If ``preparer.require_hashes`` is True, don't use the wheel cache,
+        because cached wheels, always built locally, have different hashes
+        than the files downloaded from the index server and thus throw false
+        hash mismatches. Furthermore, cached wheels at present have
+        nondeterministic contents due to file modification times.
+        """
+        if self._wheel_cache is None or self.preparer.require_hashes:
+            return None
+        return self._wheel_cache.get_cache_entry(
+            link=link,
+            package_name=name,
+            supported_tags=get_supported(),
+        )
+
+    def get_dist_to_uninstall(self, candidate):
+        # type: (Candidate) -> Optional[Distribution]
+        # TODO: Are there more cases this needs to return True? Editable?
+        dist = self._installed_dists.get(candidate.name)
+        if dist is None:  # Not installed, no uninstallation required.
+            return None
+
+        # We're installing into global site. The current installation must
+        # be uninstalled, no matter it's in global or user site, because the
+        # user site installation has precedence over global.
+        if not self._use_user_site:
+            return dist
+
+        # We're installing into user site. Remove the user site installation.
+        if dist_in_usersite(dist):
+            return dist
+
+        # We're installing into user site, but the installed incompatible
+        # package is in global site. We can't uninstall that, and would let
+        # the new user installation to "shadow" it. But shadowing won't work
+        # in virtual environments, so we error out.
+        if running_under_virtualenv() and dist_in_site_packages(dist):
+            raise InstallationError(
+                "Will not install to the user site because it will "
+                "lack sys.path precedence to {} in {}".format(
+                    dist.project_name, dist.location,
+                )
+            )
+        return None
+
+    def _report_requires_python_error(
+        self,
+        requirement,  # type: RequiresPythonRequirement
+        template,  # type: Candidate
+    ):
+        # type: (...) -> UnsupportedPythonVersion
+        message_format = (
+            "Package {package!r} requires a different Python: "
+            "{version} not in {specifier!r}"
+        )
+        message = message_format.format(
+            package=template.name,
+            version=self._python_candidate.version,
+            specifier=str(requirement.specifier),
+        )
+        return UnsupportedPythonVersion(message)
+
+    def get_installation_error(self, e):
+        # type: (ResolutionImpossible) -> InstallationError
+
+        assert e.causes, "Installation error reported with no cause"
+
+        # If one of the things we can't solve is "we need Python X.Y",
+        # that is what we report.
+        for cause in e.causes:
+            if isinstance(cause.requirement, RequiresPythonRequirement):
+                return self._report_requires_python_error(
+                    cause.requirement,
+                    cause.parent,
+                )
+
+        # Otherwise, we have a set of causes which can't all be satisfied
+        # at once.
+
+        # The simplest case is when we have *one* cause that can't be
+        # satisfied. We just report that case.
+        if len(e.causes) == 1:
+            req, parent = e.causes[0]
+            if parent is None:
+                req_disp = str(req)
+            else:
+                req_disp = '{} (from {})'.format(req, parent.name)
+            logger.critical(
+                "Could not find a version that satisfies the requirement %s",
+                req_disp,
+            )
+            return DistributionNotFound(
+                'No matching distribution found for {}'.format(req)
+            )
+
+        # OK, we now have a list of requirements that can't all be
+        # satisfied at once.
+
+        # A couple of formatting helpers
+        def text_join(parts):
+            # type: (List[str]) -> str
+            if len(parts) == 1:
+                return parts[0]
+
+            return ", ".join(parts[:-1]) + " and " + parts[-1]
+
+        def readable_form(cand):
+            # type: (Candidate) -> str
+            return "{} {}".format(cand.name, cand.version)
+
+        def describe_trigger(parent):
+            # type: (Candidate) -> str
+            ireq = parent.get_install_requirement()
+            if not ireq or not ireq.comes_from:
+                return "{} {}".format(parent.name, parent.version)
+            if isinstance(ireq.comes_from, InstallRequirement):
+                return str(ireq.comes_from.name)
+            return str(ireq.comes_from)
+
+        triggers = []
+        for req, parent in e.causes:
+            if parent is None:
+                # This is a root requirement, so we can report it directly
+                trigger = req.format_for_error()
+            else:
+                trigger = describe_trigger(parent)
+            triggers.append(trigger)
+
+        if triggers:
+            info = text_join(triggers)
+        else:
+            info = "the requested packages"
+
+        msg = "Cannot install {} because these package versions " \
+            "have conflicting dependencies.".format(info)
+        logger.critical(msg)
+        msg = "\nThe conflict is caused by:"
+        for req, parent in e.causes:
+            msg = msg + "\n    "
+            if parent:
+                msg = msg + "{} {} depends on ".format(
+                    parent.name,
+                    parent.version
+                )
+            else:
+                msg = msg + "The user requested "
+            msg = msg + req.format_for_error()
+
+        msg = msg + "\n\n" + \
+            "To fix this you could try to:\n" + \
+            "1. loosen the range of package versions you've specified\n" + \
+            "2. remove package versions to allow pip attempt to solve " + \
+            "the dependency conflict\n"
+
+        logger.info(msg)
+
+        return DistributionNotFound(
+            "ResolutionImpossible: for help visit "
+            "https://pip.pypa.io/en/latest/user_guide/"
+            "#fixing-conflicting-dependencies"
+        )

venv/Lib/site-packages/pip/_internal/resolution/resolvelib/provider.py

@@ -0,0 +1,153 @@
+from pip._vendor.packaging.specifiers import SpecifierSet
+from pip._vendor.resolvelib.providers import AbstractProvider
+
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+if MYPY_CHECK_RUNNING:
+    from typing import (
+        Any,
+        Dict,
+        Iterable,
+        Optional,
+        Sequence,
+        Set,
+        Tuple,
+        Union,
+    )
+
+    from .base import Requirement, Candidate
+    from .factory import Factory
+
+# Notes on the relationship between the provider, the factory, and the
+# candidate and requirement classes.
+#
+# The provider is a direct implementation of the resolvelib class. Its role
+# is to deliver the API that resolvelib expects.
+#
+# Rather than work with completely abstract "requirement" and "candidate"
+# concepts as resolvelib does, pip has concrete classes implementing these two
+# ideas. The API of Requirement and Candidate objects are defined in the base
+# classes, but essentially map fairly directly to the equivalent provider
+# methods. In particular, `find_matches` and `is_satisfied_by` are
+# requirement methods, and `get_dependencies` is a candidate method.
+#
+# The factory is the interface to pip's internal mechanisms. It is stateless,
+# and is created by the resolver and held as a property of the provider. It is
+# responsible for creating Requirement and Candidate objects, and provides
+# services to those objects (access to pip's finder and preparer).
+
+
+class PipProvider(AbstractProvider):
+    def __init__(
+        self,
+        factory,  # type: Factory
+        constraints,  # type: Dict[str, SpecifierSet]
+        ignore_dependencies,  # type: bool
+        upgrade_strategy,  # type: str
+        user_requested,  # type: Set[str]
+    ):
+        # type: (...) -> None
+        self._factory = factory
+        self._constraints = constraints
+        self._ignore_dependencies = ignore_dependencies
+        self._upgrade_strategy = upgrade_strategy
+        self.user_requested = user_requested
+
+    def _sort_matches(self, matches):
+        # type: (Iterable[Candidate]) -> Sequence[Candidate]
+
+        # The requirement is responsible for returning a sequence of potential
+        # candidates, one per version. The provider handles the logic of
+        # deciding the order in which these candidates should be passed to
+        # the resolver.
+
+        # The `matches` argument is a sequence of candidates, one per version,
+        # which are potential options to be installed. The requirement will
+        # have already sorted out whether to give us an already-installed
+        # candidate or a version from PyPI (i.e., it will deal with options
+        # like --force-reinstall and --ignore-installed).
+
+        # We now work out the correct order.
+        #
+        # 1. If no other considerations apply, later versions take priority.
+        # 2. An already installed distribution is preferred over any other,
+        #    unless the user has requested an upgrade.
+        #    Upgrades are allowed when:
+        #    * The --upgrade flag is set, and
+        #      - The project was specified on the command line, or
+        #      - The project is a dependency and the "eager" upgrade strategy
+        #        was requested.
+        def _eligible_for_upgrade(name):
+            # type: (str) -> bool
+            """Are upgrades allowed for this project?
+
+            This checks the upgrade strategy, and whether the project was one
+            that the user specified in the command line, in order to decide
+            whether we should upgrade if there's a newer version available.
+
+            (Note that we don't need access to the `--upgrade` flag, because
+            an upgrade strategy of "to-satisfy-only" means that `--upgrade`
+            was not specified).
+            """
+            if self._upgrade_strategy == "eager":
+                return True
+            elif self._upgrade_strategy == "only-if-needed":
+                return (name in self.user_requested)
+            return False
+
+        def sort_key(c):
+            # type: (Candidate) -> int
+            """Return a sort key for the matches.
+
+            The highest priority should be given to installed candidates that
+            are not eligible for upgrade. We use the integer value in the first
+            part of the key to sort these before other candidates.
+
+            We only pull the installed candidate to the bottom (i.e. most
+            preferred), but otherwise keep the ordering returned by the
+            requirement. The requirement is responsible for returning a list
+            otherwise sorted for the resolver, taking account for versions
+            and binary preferences as specified by the user.
+            """
+            if c.is_installed and not _eligible_for_upgrade(c.name):
+                return 1
+            return 0
+
+        return sorted(matches, key=sort_key)
+
+    def identify(self, dependency):
+        # type: (Union[Requirement, Candidate]) -> str
+        return dependency.name
+
+    def get_preference(
+        self,
+        resolution,  # type: Optional[Candidate]
+        candidates,  # type: Sequence[Candidate]
+        information  # type: Sequence[Tuple[Requirement, Candidate]]
+    ):
+        # type: (...) -> Any
+        # Use the "usual" value for now
+        return len(candidates)
+
+    def find_matches(self, requirements):
+        # type: (Sequence[Requirement]) -> Iterable[Candidate]
+        if not requirements:
+            return []
+        constraint = self._constraints.get(
+            requirements[0].name, SpecifierSet(),
+        )
+        candidates = self._factory.find_candidates(requirements, constraint)
+        return reversed(self._sort_matches(candidates))
+
+    def is_satisfied_by(self, requirement, candidate):
+        # type: (Requirement, Candidate) -> bool
+        return requirement.is_satisfied_by(candidate)
+
+    def get_dependencies(self, candidate):
+        # type: (Candidate) -> Sequence[Requirement]
+        with_requires = not self._ignore_dependencies
+        return [
+            r
+            for r in candidate.iter_dependencies(with_requires)
+            if r is not None
+        ]

venv/Lib/site-packages/pip/_internal/resolution/resolvelib/requirements.py

@@ -0,0 +1,137 @@
+from pip._vendor.packaging.utils import canonicalize_name
+
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+from .base import Requirement, format_name
+
+if MYPY_CHECK_RUNNING:
+    from pip._vendor.packaging.specifiers import SpecifierSet
+
+    from pip._internal.req.req_install import InstallRequirement
+
+    from .base import Candidate, CandidateLookup
+
+
+class ExplicitRequirement(Requirement):
+    def __init__(self, candidate):
+        # type: (Candidate) -> None
+        self.candidate = candidate
+
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}({candidate!r})".format(
+            class_name=self.__class__.__name__,
+            candidate=self.candidate,
+        )
+
+    @property
+    def name(self):
+        # type: () -> str
+        # No need to canonicalise - the candidate did this
+        return self.candidate.name
+
+    def format_for_error(self):
+        # type: () -> str
+        return self.candidate.format_for_error()
+
+    def get_candidate_lookup(self):
+        # type: () -> CandidateLookup
+        return self.candidate, None
+
+    def is_satisfied_by(self, candidate):
+        # type: (Candidate) -> bool
+        return candidate == self.candidate
+
+
+class SpecifierRequirement(Requirement):
+    def __init__(self, ireq):
+        # type: (InstallRequirement) -> None
+        assert ireq.link is None, "This is a link, not a specifier"
+        self._ireq = ireq
+        self._extras = frozenset(ireq.extras)
+
+    def __str__(self):
+        # type: () -> str
+        return str(self._ireq.req)
+
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}({requirement!r})".format(
+            class_name=self.__class__.__name__,
+            requirement=str(self._ireq.req),
+        )
+
+    @property
+    def name(self):
+        # type: () -> str
+        canonical_name = canonicalize_name(self._ireq.req.name)
+        return format_name(canonical_name, self._extras)
+
+    def format_for_error(self):
+        # type: () -> str
+
+        # Convert comma-separated specifiers into "A, B, ..., F and G"
+        # This makes the specifier a bit more "human readable", without
+        # risking a change in meaning. (Hopefully! Not all edge cases have
+        # been checked)
+        parts = [s.strip() for s in str(self).split(",")]
+        if len(parts) == 0:
+            return ""
+        elif len(parts) == 1:
+            return parts[0]
+
+        return ", ".join(parts[:-1]) + " and " + parts[-1]
+
+    def get_candidate_lookup(self):
+        # type: () -> CandidateLookup
+        return None, self._ireq
+
+    def is_satisfied_by(self, candidate):
+        # type: (Candidate) -> bool
+        assert candidate.name == self.name, \
+            "Internal issue: Candidate is not for this requirement " \
+            " {} vs {}".format(candidate.name, self.name)
+        # We can safely always allow prereleases here since PackageFinder
+        # already implements the prerelease logic, and would have filtered out
+        # prerelease candidates if the user does not expect them.
+        spec = self._ireq.req.specifier
+        return spec.contains(candidate.version, prereleases=True)
+
+
+class RequiresPythonRequirement(Requirement):
+    """A requirement representing Requires-Python metadata.
+    """
+    def __init__(self, specifier, match):
+        # type: (SpecifierSet, Candidate) -> None
+        self.specifier = specifier
+        self._candidate = match
+
+    def __repr__(self):
+        # type: () -> str
+        return "{class_name}({specifier!r})".format(
+            class_name=self.__class__.__name__,
+            specifier=str(self.specifier),
+        )
+
+    @property
+    def name(self):
+        # type: () -> str
+        return self._candidate.name
+
+    def format_for_error(self):
+        # type: () -> str
+        return "Python " + str(self.specifier)
+
+    def get_candidate_lookup(self):
+        # type: () -> CandidateLookup
+        if self.specifier.contains(self._candidate.version, prereleases=True):
+            return self._candidate, None
+        return None, None
+
+    def is_satisfied_by(self, candidate):
+        # type: (Candidate) -> bool
+        assert candidate.name == self._candidate.name, "Not Python candidate"
+        # We can safely always allow prereleases here since PackageFinder
+        # already implements the prerelease logic, and would have filtered out
+        # prerelease candidates if the user does not expect them.
+        return self.specifier.contains(candidate.version, prereleases=True)

venv/Lib/site-packages/pip/_internal/resolution/resolvelib/resolver.py

@@ -0,0 +1,259 @@
+import functools
+import logging
+
+from pip._vendor import six
+from pip._vendor.packaging.utils import canonicalize_name
+from pip._vendor.resolvelib import BaseReporter, ResolutionImpossible
+from pip._vendor.resolvelib import Resolver as RLResolver
+
+from pip._internal.exceptions import InstallationError
+from pip._internal.req.req_install import check_invalid_constraint_type
+from pip._internal.req.req_set import RequirementSet
+from pip._internal.resolution.base import BaseResolver
+from pip._internal.resolution.resolvelib.provider import PipProvider
+from pip._internal.utils.misc import dist_is_editable
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+from .factory import Factory
+
+if MYPY_CHECK_RUNNING:
+    from typing import Dict, List, Optional, Set, Tuple
+
+    from pip._vendor.packaging.specifiers import SpecifierSet
+    from pip._vendor.resolvelib.resolvers import Result
+    from pip._vendor.resolvelib.structs import Graph
+
+    from pip._internal.cache import WheelCache
+    from pip._internal.index.package_finder import PackageFinder
+    from pip._internal.operations.prepare import RequirementPreparer
+    from pip._internal.req.req_install import InstallRequirement
+    from pip._internal.resolution.base import InstallRequirementProvider
+
+
+logger = logging.getLogger(__name__)
+
+
+class Resolver(BaseResolver):
+    _allowed_strategies = {"eager", "only-if-needed", "to-satisfy-only"}
+
+    def __init__(
+        self,
+        preparer,  # type: RequirementPreparer
+        finder,  # type: PackageFinder
+        wheel_cache,  # type: Optional[WheelCache]
+        make_install_req,  # type: InstallRequirementProvider
+        use_user_site,  # type: bool
+        ignore_dependencies,  # type: bool
+        ignore_installed,  # type: bool
+        ignore_requires_python,  # type: bool
+        force_reinstall,  # type: bool
+        upgrade_strategy,  # type: str
+        py_version_info=None,  # type: Optional[Tuple[int, ...]]
+        lazy_wheel=False,  # type: bool
+    ):
+        super(Resolver, self).__init__()
+        if lazy_wheel:
+            logger.warning(
+                'pip is using lazily downloaded wheels using HTTP '
+                'range requests to obtain dependency information. '
+                'This experimental feature is enabled through '
+                '--use-feature=fast-deps and it is not ready for production.'
+            )
+
+        assert upgrade_strategy in self._allowed_strategies
+
+        self.factory = Factory(
+            finder=finder,
+            preparer=preparer,
+            make_install_req=make_install_req,
+            wheel_cache=wheel_cache,
+            use_user_site=use_user_site,
+            force_reinstall=force_reinstall,
+            ignore_installed=ignore_installed,
+            ignore_requires_python=ignore_requires_python,
+            py_version_info=py_version_info,
+            lazy_wheel=lazy_wheel,
+        )
+        self.ignore_dependencies = ignore_dependencies
+        self.upgrade_strategy = upgrade_strategy
+        self._result = None  # type: Optional[Result]
+
+    def resolve(self, root_reqs, check_supported_wheels):
+        # type: (List[InstallRequirement], bool) -> RequirementSet
+
+        constraints = {}  # type: Dict[str, SpecifierSet]
+        user_requested = set()  # type: Set[str]
+        requirements = []
+        for req in root_reqs:
+            if req.constraint:
+                # Ensure we only accept valid constraints
+                problem = check_invalid_constraint_type(req)
+                if problem:
+                    raise InstallationError(problem)
+                if not req.match_markers():
+                    continue
+                name = canonicalize_name(req.name)
+                if name in constraints:
+                    constraints[name] = constraints[name] & req.specifier
+                else:
+                    constraints[name] = req.specifier
+            else:
+                if req.user_supplied and req.name:
+                    user_requested.add(canonicalize_name(req.name))
+                r = self.factory.make_requirement_from_install_req(
+                    req, requested_extras=(),
+                )
+                if r is not None:
+                    requirements.append(r)
+
+        provider = PipProvider(
+            factory=self.factory,
+            constraints=constraints,
+            ignore_dependencies=self.ignore_dependencies,
+            upgrade_strategy=self.upgrade_strategy,
+            user_requested=user_requested,
+        )
+        reporter = BaseReporter()
+        resolver = RLResolver(provider, reporter)
+
+        try:
+            try_to_avoid_resolution_too_deep = 2000000
+            self._result = resolver.resolve(
+                requirements, max_rounds=try_to_avoid_resolution_too_deep,
+            )
+
+        except ResolutionImpossible as e:
+            error = self.factory.get_installation_error(e)
+            six.raise_from(error, e)
+
+        req_set = RequirementSet(check_supported_wheels=check_supported_wheels)
+        for candidate in self._result.mapping.values():
+            ireq = candidate.get_install_requirement()
+            if ireq is None:
+                continue
+
+            # Check if there is already an installation under the same name,
+            # and set a flag for later stages to uninstall it, if needed.
+            # * There isn't, good -- no uninstalltion needed.
+            # * The --force-reinstall flag is set. Always reinstall.
+            # * The installation is different in version or editable-ness, so
+            #   we need to uninstall it to install the new distribution.
+            # * The installed version is the same as the pending distribution.
+            #   Skip this distrubiton altogether to save work.
+            installed_dist = self.factory.get_dist_to_uninstall(candidate)
+            if installed_dist is None:
+                ireq.should_reinstall = False
+            elif self.factory.force_reinstall:
+                ireq.should_reinstall = True
+            elif installed_dist.parsed_version != candidate.version:
+                ireq.should_reinstall = True
+            elif dist_is_editable(installed_dist) != candidate.is_editable:
+                ireq.should_reinstall = True
+            else:
+                continue
+
+            link = candidate.source_link
+            if link and link.is_yanked:
+                # The reason can contain non-ASCII characters, Unicode
+                # is required for Python 2.
+                msg = (
+                    u'The candidate selected for download or install is a '
+                    u'yanked version: {name!r} candidate (version {version} '
+                    u'at {link})\nReason for being yanked: {reason}'
+                ).format(
+                    name=candidate.name,
+                    version=candidate.version,
+                    link=link,
+                    reason=link.yanked_reason or u'<none given>',
+                )
+                logger.warning(msg)
+
+            req_set.add_named_requirement(ireq)
+
+        return req_set
+
+    def get_installation_order(self, req_set):
+        # type: (RequirementSet) -> List[InstallRequirement]
+        """Get order for installation of requirements in RequirementSet.
+
+        The returned list contains a requirement before another that depends on
+        it. This helps ensure that the environment is kept consistent as they
+        get installed one-by-one.
+
+        The current implementation creates a topological ordering of the
+        dependency graph, while breaking any cycles in the graph at arbitrary
+        points. We make no guarantees about where the cycle would be broken,
+        other than they would be broken.
+        """
+        assert self._result is not None, "must call resolve() first"
+
+        graph = self._result.graph
+        weights = get_topological_weights(graph)
+
+        sorted_items = sorted(
+            req_set.requirements.items(),
+            key=functools.partial(_req_set_item_sorter, weights=weights),
+            reverse=True,
+        )
+        return [ireq for _, ireq in sorted_items]
+
+
+def get_topological_weights(graph):
+    # type: (Graph) -> Dict[Optional[str], int]
+    """Assign weights to each node based on how "deep" they are.
+
+    This implementation may change at any point in the future without prior
+    notice.
+
+    We take the length for the longest path to any node from root, ignoring any
+    paths that contain a single node twice (i.e. cycles). This is done through
+    a depth-first search through the graph, while keeping track of the path to
+    the node.
+
+    Cycles in the graph result would result in node being revisited while also
+    being it's own path. In this case, take no action. This helps ensure we
+    don't get stuck in a cycle.
+
+    When assigning weight, the longer path (i.e. larger length) is preferred.
+    """
+    path = set()  # type: Set[Optional[str]]
+    weights = {}  # type: Dict[Optional[str], int]
+
+    def visit(node):
+        # type: (Optional[str]) -> None
+        if node in path:
+            # We hit a cycle, so we'll break it here.
+            return
+
+        # Time to visit the children!
+        path.add(node)
+        for child in graph.iter_children(node):
+            visit(child)
+        path.remove(node)
+
+        last_known_parent_count = weights.get(node, 0)
+        weights[node] = max(last_known_parent_count, len(path))
+
+    # `None` is guaranteed to be the root node by resolvelib.
+    visit(None)
+
+    # Sanity checks
+    assert weights[None] == 0
+    assert len(weights) == len(graph)
+
+    return weights
+
+
+def _req_set_item_sorter(
+    item,     # type: Tuple[str, InstallRequirement]
+    weights,  # type: Dict[Optional[str], int]
+):
+    # type: (...) -> Tuple[int, str]
+    """Key function used to sort install requirements for installation.
+
+    Based on the "weight" mapping calculated in ``get_installation_order()``.
+    The canonical package name is returned as the second member as a tie-
+    breaker to ensure the result is predictable, which is useful in tests.
+    """
+    name = canonicalize_name(item[0])
+    return weights[name], name