_py_abc.py

from_weakrefsetimportWeakSet


defget_cache_token():
"""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.
 """
returnABCMeta._abc_invalidation_counter


classABCMeta(type):
"""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
 even via super()).
 """

# 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
# external code.
_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
abstracts= {name
forname, valueinnamespace.items()
ifgetattr(value, "__isabstractmethod__", False)}
forbaseinbases:
fornameingetattr(base, "__abstractmethods__", set()):
value=getattr(cls, name, None)
ifgetattr(value, "__isabstractmethod__", False):
abstracts.add(name)
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
returncls

defregister(cls, subclass):
"""Register a virtual subclass of an ABC.

 Returns the subclass, to allow usage as a class decorator.
 """
ifnotisinstance(subclass, type):
raiseTypeError("Can only register classes")
ifissubclass(subclass, cls):
returnsubclass# 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.
ifissubclass(cls, subclass):
# This would create a cycle, which is bad for the algorithm below
raiseRuntimeError("Refusing to create an inheritance cycle")
cls._abc_registry.add(subclass)
ABCMeta._abc_invalidation_counter+=1# Invalidate negative cache
returnsubclass

def_dump_registry(cls, file=None):
"""Debug helper to print the ABC registry."""
print(f"Class: {cls.__module__}.{cls.__qualname__}", file=file)
print(f"Inv. counter: {get_cache_token()}", file=file)
fornameincls.__dict__:
ifname.startswith("_abc_"):
value=getattr(cls, name)
ifisinstance(value, WeakSet):
value=set(value)
print(f"{name}: {value!r}", file=file)

def_abc_registry_clear(cls):
"""Clear the registry (for debugging or testing)."""
cls._abc_registry.clear()

def_abc_caches_clear(cls):
"""Clear the caches (for debugging or testing)."""
cls._abc_cache.clear()
cls._abc_negative_cache.clear()

def__instancecheck__(cls, instance):
"""Override for isinstance(instance, cls)."""
# Inline the cache checking
subclass=instance.__class__
ifsubclassincls._abc_cache:
returnTrue
subtype=type(instance)
ifsubtypeissubclass:
if (cls._abc_negative_cache_version==
ABCMeta._abc_invalidation_counterand
subclassincls._abc_negative_cache):
returnFalse
# Fall back to the subclass check.
returncls.__subclasscheck__(subclass)
returnany(cls.__subclasscheck__(c) forcin (subclass, subtype))

def__subclasscheck__(cls, subclass):
"""Override for issubclass(subclass, cls)."""
ifnotisinstance(subclass, type):
raiseTypeError('issubclass() arg 1 must be a class')
# Check cache
ifsubclassincls._abc_cache:
returnTrue
# Check negative cache; may have to invalidate
ifcls._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
elifsubclassincls._abc_negative_cache:
returnFalse
# Check the subclass hook
ok=cls.__subclasshook__(subclass)
ifokisnotNotImplemented:
assertisinstance(ok, bool)
ifok:
cls._abc_cache.add(subclass)
else:
cls._abc_negative_cache.add(subclass)
returnok
# Check if it's a direct subclass
ifclsingetattr(subclass, '__mro__', ()):
cls._abc_cache.add(subclass)
returnTrue
# Check if it's a subclass of a registered class (recursive)
forrclsincls._abc_registry:
ifissubclass(subclass, rcls):
cls._abc_cache.add(subclass)
returnTrue
# Check if it's a subclass of a subclass (recursive)
forsclsincls.__subclasses__():
ifissubclass(subclass, scls):
cls._abc_cache.add(subclass)
returnTrue
# No dice; update negative cache
cls._abc_negative_cache.add(subclass)
returnFalse