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185 lines
7.0 KiB
185 lines
7.0 KiB
# 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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import types
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from _weakrefset import WeakSet
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# Instance of old-style class
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class _C: pass
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_InstanceType = type(_C())
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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:
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__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 abstractproperty(property):
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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:
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__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:
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__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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"""
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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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_abc_invalidation_counter = 0
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def __new__(mcls, name, bases, namespace):
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cls = super(ABCMeta, mcls).__new__(mcls, name, bases, namespace)
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# Compute set of abstract method names
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abstracts = set(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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if not isinstance(subclass, (type, types.ClassType)):
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raise TypeError("Can only register classes")
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if issubclass(subclass, cls):
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return # 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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def _dump_registry(cls, file=None):
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"""Debug helper to print the ABC registry."""
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print >> file, "Class: %s.%s" % (cls.__module__, cls.__name__)
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print >> file, "Inv.counter: %s" % ABCMeta._abc_invalidation_counter
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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 >> file, "%s: %r" % (name, value)
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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 when it's simple.
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subclass = getattr(instance, '__class__', None)
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if subclass is not None and subclass in cls._abc_cache:
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return True
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subtype = type(instance)
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# Old-style instances
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if subtype is _InstanceType:
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subtype = subclass
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if subtype is subclass or subclass is None:
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if (cls._abc_negative_cache_version ==
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ABCMeta._abc_invalidation_counter and
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subtype 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__(subtype)
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return (cls.__subclasscheck__(subclass) or
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cls.__subclasscheck__(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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