"""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.

"""

"""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

for name, value in namespace.items()

if getattr(value, "__isabstractmethod__", False)}

for base in bases:

for name in getattr(base, "__abstractmethods__", set()):

value = getattr(cls, name, None)

if getattr(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

return cls

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

return subclass

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)

for name in cls.__dict__:

if name.startswith("_abc_"):

value = getattr(cls, name)

if isinstance(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__

if subclass in cls._abc_cache:

return True

subtype = type(instance)

if subtype is subclass:

if (cls._abc_negative_cache_version ==

ABCMeta._abc_invalidation_counter and

subclass in cls._abc_negative_cache):

return False

# 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 not isinstance(subclass, type):

raise TypeError('issubclass() arg 1 must be a class')

# Check cache

if subclass in cls._abc_cache:

return True

# 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:

return False

# Check the subclass hook

ok = cls.__subclasshook__(subclass)

if ok is not NotImplemented:

assert isinstance(ok, bool)

if ok:

cls._abc_cache.add(subclass)

else:

cls._abc_negative_cache.add(subclass)

return ok

# Check if it's a direct subclass

if cls in getattr(subclass, '__mro__', ()):

cls._abc_cache.add(subclass)

return True

# 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)

return True

# Check if it's a subclass of a subclass (recursive)

for scls in cls.__subclasses__():

if issubclass(subclass, scls):

cls._abc_cache.add(subclass)

return True

# No dice; update negative cache

cls._abc_negative_cache.add(subclass)