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248 lines
8.4 KiB
Python
248 lines
8.4 KiB
Python
# Copyright 2007 Google, Inc. All Rights Reserved.
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# Licensed to PSF under a Contributor Agreement.
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"""Abstract Base Classes (ABCs) according to PEP 3119."""
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from _weakrefset import WeakSet
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def abstractmethod(funcobj):
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"""A decorator indicating abstract methods.
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Requires that the metaclass is ABCMeta or derived from it. A
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class that has a metaclass derived from ABCMeta cannot be
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instantiated unless all of its abstract methods are overridden.
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The abstract methods can be called using any of the normal
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'super' call mechanisms.
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Usage:
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class C(metaclass=ABCMeta):
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@abstractmethod
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def my_abstract_method(self, ...):
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...
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"""
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funcobj.__isabstractmethod__ = True
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return funcobj
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class abstractclassmethod(classmethod):
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"""
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A decorator indicating abstract classmethods.
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Similar to abstractmethod.
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Usage:
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class C(metaclass=ABCMeta):
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@abstractclassmethod
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def my_abstract_classmethod(cls, ...):
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...
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'abstractclassmethod' is deprecated. Use 'classmethod' with
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'abstractmethod' instead.
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"""
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__isabstractmethod__ = True
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def __init__(self, callable):
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callable.__isabstractmethod__ = True
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super().__init__(callable)
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class abstractstaticmethod(staticmethod):
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"""
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A decorator indicating abstract staticmethods.
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Similar to abstractmethod.
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Usage:
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class C(metaclass=ABCMeta):
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@abstractstaticmethod
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def my_abstract_staticmethod(...):
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...
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'abstractstaticmethod' is deprecated. Use 'staticmethod' with
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'abstractmethod' instead.
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"""
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__isabstractmethod__ = True
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def __init__(self, callable):
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callable.__isabstractmethod__ = True
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super().__init__(callable)
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class abstractproperty(property):
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"""
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A decorator indicating abstract properties.
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Requires that the metaclass is ABCMeta or derived from it. A
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class that has a metaclass derived from ABCMeta cannot be
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instantiated unless all of its abstract properties are overridden.
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The abstract properties can be called using any of the normal
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'super' call mechanisms.
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Usage:
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class C(metaclass=ABCMeta):
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@abstractproperty
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def my_abstract_property(self):
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...
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This defines a read-only property; you can also define a read-write
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abstract property using the 'long' form of property declaration:
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class C(metaclass=ABCMeta):
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def getx(self): ...
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def setx(self, value): ...
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x = abstractproperty(getx, setx)
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'abstractproperty' is deprecated. Use 'property' with 'abstractmethod'
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instead.
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"""
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__isabstractmethod__ = True
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class ABCMeta(type):
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"""Metaclass for defining Abstract Base Classes (ABCs).
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Use this metaclass to create an ABC. An ABC can be subclassed
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directly, and then acts as a mix-in class. You can also register
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unrelated concrete classes (even built-in classes) and unrelated
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ABCs as 'virtual subclasses' -- these and their descendants will
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be considered subclasses of the registering ABC by the built-in
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issubclass() function, but the registering ABC won't show up in
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their MRO (Method Resolution Order) nor will method
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implementations defined by the registering ABC be callable (not
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even via super()).
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"""
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# A global counter that is incremented each time a class is
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# registered as a virtual subclass of anything. It forces the
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# negative cache to be cleared before its next use.
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# Note: this counter is private. Use `abc.get_cache_token()` for
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# external code.
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_abc_invalidation_counter = 0
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def __new__(mcls, name, bases, namespace):
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cls = super().__new__(mcls, name, bases, namespace)
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# Compute set of abstract method names
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abstracts = {name
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for name, value in namespace.items()
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if getattr(value, "__isabstractmethod__", False)}
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for base in bases:
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for name in getattr(base, "__abstractmethods__", set()):
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value = getattr(cls, name, None)
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if getattr(value, "__isabstractmethod__", False):
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abstracts.add(name)
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cls.__abstractmethods__ = frozenset(abstracts)
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# Set up inheritance registry
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cls._abc_registry = WeakSet()
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cls._abc_cache = WeakSet()
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cls._abc_negative_cache = WeakSet()
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cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
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return cls
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def register(cls, subclass):
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"""Register a virtual subclass of an ABC.
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Returns the subclass, to allow usage as a class decorator.
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"""
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if not isinstance(subclass, type):
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raise TypeError("Can only register classes")
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if issubclass(subclass, cls):
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return subclass # Already a subclass
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# Subtle: test for cycles *after* testing for "already a subclass";
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# this means we allow X.register(X) and interpret it as a no-op.
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if issubclass(cls, subclass):
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# This would create a cycle, which is bad for the algorithm below
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raise RuntimeError("Refusing to create an inheritance cycle")
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cls._abc_registry.add(subclass)
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ABCMeta._abc_invalidation_counter += 1 # Invalidate negative cache
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return subclass
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def _dump_registry(cls, file=None):
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"""Debug helper to print the ABC registry."""
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print("Class: %s.%s" % (cls.__module__, cls.__qualname__), file=file)
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print("Inv.counter: %s" % ABCMeta._abc_invalidation_counter, file=file)
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for name in sorted(cls.__dict__.keys()):
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if name.startswith("_abc_"):
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value = getattr(cls, name)
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print("%s: %r" % (name, value), file=file)
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def __instancecheck__(cls, instance):
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"""Override for isinstance(instance, cls)."""
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# Inline the cache checking
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subclass = instance.__class__
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if subclass in cls._abc_cache:
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return True
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subtype = type(instance)
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if subtype is subclass:
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if (cls._abc_negative_cache_version ==
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ABCMeta._abc_invalidation_counter and
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subclass in cls._abc_negative_cache):
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return False
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# Fall back to the subclass check.
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return cls.__subclasscheck__(subclass)
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return any(cls.__subclasscheck__(c) for c in {subclass, subtype})
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def __subclasscheck__(cls, subclass):
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"""Override for issubclass(subclass, cls)."""
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# Check cache
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if subclass in cls._abc_cache:
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return True
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# Check negative cache; may have to invalidate
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if cls._abc_negative_cache_version < ABCMeta._abc_invalidation_counter:
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# Invalidate the negative cache
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cls._abc_negative_cache = WeakSet()
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cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
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elif subclass in cls._abc_negative_cache:
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return False
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# Check the subclass hook
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ok = cls.__subclasshook__(subclass)
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if ok is not NotImplemented:
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assert isinstance(ok, bool)
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if ok:
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cls._abc_cache.add(subclass)
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else:
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cls._abc_negative_cache.add(subclass)
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return ok
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# Check if it's a direct subclass
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if cls in getattr(subclass, '__mro__', ()):
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cls._abc_cache.add(subclass)
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return True
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# Check if it's a subclass of a registered class (recursive)
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for rcls in cls._abc_registry:
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if issubclass(subclass, rcls):
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cls._abc_cache.add(subclass)
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return True
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# Check if it's a subclass of a subclass (recursive)
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for scls in cls.__subclasses__():
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if issubclass(subclass, scls):
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cls._abc_cache.add(subclass)
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return True
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# No dice; update negative cache
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cls._abc_negative_cache.add(subclass)
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return False
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class ABC(metaclass=ABCMeta):
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"""Helper class that provides a standard way to create an ABC using
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inheritance.
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"""
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pass
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def get_cache_token():
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"""Returns the current ABC cache token.
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The token is an opaque object (supporting equality testing) identifying the
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current version of the ABC cache for virtual subclasses. The token changes
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with every call to ``register()`` on any ABC.
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"""
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return ABCMeta._abc_invalidation_counter
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