104 lines
3.5 KiB
Python
104 lines
3.5 KiB
Python
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import sys
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def immutable(members='', name='Immutable', verbose=False):
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"""
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Produces a class that either can be used standalone or as a base class for persistent classes.
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This is a thin wrapper around a named tuple.
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Constructing a type and using it to instantiate objects:
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>>> Point = immutable('x, y', name='Point')
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>>> p = Point(1, 2)
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>>> p2 = p.set(x=3)
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>>> p
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Point(x=1, y=2)
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>>> p2
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Point(x=3, y=2)
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Inheriting from a constructed type. In this case no type name needs to be supplied:
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>>> class PositivePoint(immutable('x, y')):
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... __slots__ = tuple()
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... def __new__(cls, x, y):
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... if x > 0 and y > 0:
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... return super(PositivePoint, cls).__new__(cls, x, y)
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... raise Exception('Coordinates must be positive!')
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...
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>>> p = PositivePoint(1, 2)
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>>> p.set(x=3)
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PositivePoint(x=3, y=2)
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>>> p.set(y=-3)
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Traceback (most recent call last):
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Exception: Coordinates must be positive!
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The persistent class also supports the notion of frozen members. The value of a frozen member
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cannot be updated. For example it could be used to implement an ID that should remain the same
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over time. A frozen member is denoted by a trailing underscore.
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>>> Point = immutable('x, y, id_', name='Point')
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>>> p = Point(1, 2, id_=17)
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>>> p.set(x=3)
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Point(x=3, y=2, id_=17)
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>>> p.set(id_=18)
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Traceback (most recent call last):
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AttributeError: Cannot set frozen members id_
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"""
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if isinstance(members, str):
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members = members.replace(',', ' ').split()
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def frozen_member_test():
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frozen_members = ["'%s'" % f for f in members if f.endswith('_')]
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if frozen_members:
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return """
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frozen_fields = fields_to_modify & set([{frozen_members}])
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if frozen_fields:
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raise AttributeError('Cannot set frozen members %s' % ', '.join(frozen_fields))
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""".format(frozen_members=', '.join(frozen_members))
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return ''
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verbose_string = ""
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if sys.version_info < (3, 7):
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# Verbose is no longer supported in Python 3.7
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verbose_string = ", verbose={verbose}".format(verbose=verbose)
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quoted_members = ', '.join("'%s'" % m for m in members)
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template = """
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class {class_name}(namedtuple('ImmutableBase', [{quoted_members}]{verbose_string})):
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__slots__ = tuple()
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def __repr__(self):
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return super({class_name}, self).__repr__().replace('ImmutableBase', self.__class__.__name__)
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def set(self, **kwargs):
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if not kwargs:
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return self
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fields_to_modify = set(kwargs.keys())
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if not fields_to_modify <= {member_set}:
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raise AttributeError("'%s' is not a member" % ', '.join(fields_to_modify - {member_set}))
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{frozen_member_test}
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return self.__class__.__new__(self.__class__, *map(kwargs.pop, [{quoted_members}], self))
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""".format(quoted_members=quoted_members,
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member_set="set([%s])" % quoted_members if quoted_members else 'set()',
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frozen_member_test=frozen_member_test(),
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verbose_string=verbose_string,
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class_name=name)
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if verbose:
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print(template)
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from collections import namedtuple
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namespace = dict(namedtuple=namedtuple, __name__='pyrsistent_immutable')
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try:
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exec(template, namespace)
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except SyntaxError as e:
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raise SyntaxError(e.message + ':\n' + template) from e
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return namespace[name]
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