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