這一章的內容基本上都是容易理解的。在一邊讀一邊敲代碼的過程中，發現了一個問題。
在 tombola_runner.py (在書中是示例 11-15) 中有這樣一行代碼：
virtual_subclasses = list(Tombola._abc_registry)
運行會報錯：
AttributeError: type object 'Tombola' has no attribute '_abc_registry'
經過多次排查，最終發現問題出在了 Python 版本上。我的 Python 版本是 3.8，它是不支持抽象基類中的 _abc_registry 這一數據屬性的。作者用的版本似乎是 Python 3.4，所以我借來了同學的電腦，配置了 Python 3.4 的環境，然后測試，發現可以正常運行：
下面就通過查看源碼的方式來稍微探究一下 Python 3.8 和 Python 3.4 的區別（當然其他的高版本的 Python 可能也有這樣的問題），這里用到工具是 PyCharm。
在 Python 3.4 中，abc.py 的源碼是這樣的：
# 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
'super' call mechanisms.
Usage:
class C(metaclass=ABCMeta):
@abstractmethod
def my_abstract_method(self, ...):
...
"""
funcobj.__isabstractmethod__ = True
return funcobj
class abstractclassmethod(classmethod):
"""
A decorator indicating abstract classmethods.
Similar to abstractmethod.
Usage:
class C(metaclass=ABCMeta):
@abstractclassmethod
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.
Usage:
class C(metaclass=ABCMeta):
@abstractstaticmethod
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
'super' call mechanisms.
Usage:
class C(metaclass=ABCMeta):
@abstractproperty
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 getx(self): ...
def setx(self, value): ...
x = abstractproperty(getx, setx)
'abstractproperty' is deprecated. Use 'property' with 'abstractmethod'
instead.
"""
__isabstractmethod__ = True
class ABCMeta(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):
cls = super().__new__(mcls, name, bases, namespace)
# 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() # ②
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) # ①
ABCMeta._abc_invalidation_counter += 1 # Invalidate negative cache
return subclass
def _dump_registry(cls, file=None):
"""Debug helper to print the ABC registry."""
print("Class: %s.%s" % (cls.__module__, cls.__name__), file=file)
print("Inv.counter: %s" % ABCMeta._abc_invalidation_counter, file=file)
for name in sorted(cls.__dict__.keys()):
if name.startswith("_abc_"):
value = getattr(cls, name)
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:
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)."""
# 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)
return False
class ABC(metaclass=ABCMeta):
"""Helper class that provides a standard way to create an ABC using
inheritance.
"""
pass
def get_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.
"""
return ABCMeta._abc_invalidation_counter
① 在代碼中標記 “①” 的地方，我們可以看到這樣一行代碼：
cls._abc_registry.add(subclass)
這行代碼是在 register(cls, subclass) 方法體中，很顯然，它表明了，在注冊虛擬子類時，該函數將每一個相應的虛擬子類（subclass）加入了 _abc_registry 屬性中，而這個屬性，是屬於抽象基類（cls）的。
② 從 __new__() 方法中，我們可以看到 _abc_registry 屬性被賦值：
cls._abc_cache = WeakSet()
我們再來看 Python 3.8 中的 abc.py 源碼：
# Copyright 2007 Google, Inc. All Rights Reserved.
# Licensed to PSF under a Contributor Agreement.
"""Abstract Base Classes (ABCs) according to PEP 3119."""
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
'super' call mechanisms. abstractmethod() may be used to declare
abstract methods for properties and descriptors.
Usage:
class C(metaclass=ABCMeta):
@abstractmethod
def my_abstract_method(self, ...):
...
"""
funcobj.__isabstractmethod__ = True
return funcobj
class abstractclassmethod(classmethod):
"""A decorator indicating abstract classmethods.
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.
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.
Deprecated, use 'property' with 'abstractmethod' instead.
"""
__isabstractmethod__ = True
try:
from _abc import (get_cache_token, _abc_init, _abc_register,
_abc_instancecheck, _abc_subclasscheck, _get_dump,
_reset_registry, _reset_caches)
except ImportError:
from _py_abc import ABCMeta, get_cache_token
ABCMeta.__module__ = 'abc'
else:
class ABCMeta(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()).
"""
def __new__(mcls, name, bases, namespace, **kwargs): # ①
cls = super().__new__(mcls, name, bases, namespace, **kwargs)
_abc_init(cls)
return cls
def register(cls, subclass):
"""Register a virtual subclass of an ABC.
Returns the subclass, to allow usage as a class decorator.
"""
return _abc_register(cls, subclass)
def __instancecheck__(cls, instance):
"""Override for isinstance(instance, cls)."""
return _abc_instancecheck(cls, instance)
def __subclasscheck__(cls, subclass):
"""Override for issubclass(subclass, cls)."""
return _abc_subclasscheck(cls, subclass)
def _dump_registry(cls, file=None): # ②
"""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)
(_abc_registry, _abc_cache, _abc_negative_cache,
_abc_negative_cache_version) = _get_dump(cls)
print(f"_abc_registry: {_abc_registry!r}", file=file)
print(f"_abc_cache: {_abc_cache!r}", file=file)
print(f"_abc_negative_cache: {_abc_negative_cache!r}", file=file)
print(f"_abc_negative_cache_version: {_abc_negative_cache_version!r}",
file=file)
def _abc_registry_clear(cls):
"""Clear the registry (for debugging or testing)."""
_reset_registry(cls)
def _abc_caches_clear(cls):
"""Clear the caches (for debugging or testing)."""
_reset_caches(cls)
class ABC(metaclass=ABCMeta):
"""Helper class that provides a standard way to create an ABC using
inheritance.
"""
__slots__ = ()
① 我們再來觀察這個源碼的 __new__() 方法，發現它的實現被封裝好的 _abc_init() 函數給隱藏起來了，那我們再進一步查看 _abc_init() 方法：
def _abc_init(*args, **kwargs): # real signature unknown
""" Internal ABC helper for class set-up. Should be never used outside abc module. """
pass
可以發現，它的注釋說，真實的簽名是未知的，並且，它的文檔注釋也說明了一點，這個函數的內容是不會在 abc 這個模塊的外面被使用的。也就是說，在我們看不到里面具體實現的情況下，我們不清楚抽象基類是否會有 _abc_registry 這個屬性，即使有這個屬性，它也不能夠被我們訪問到。事實上，這個 _abc_registry 屬性確實還是在的，我們可以看上面的代碼的 ② 部分。
② 這個函數是 _dump_registry()，里面有這樣的語句：
print(f"Inv. counter: {get_cache_token()}", file=file)
(_abc_registry, _abc_cache, _abc_negative_cache,
_abc_negative_cache_version) = _get_dump(cls)
print(f"_abc_registry: {_abc_registry!r}", file=file)
我們在抽象基類上調用這個方法：
Tombola._dump_registry()
打印的結果如下：
Inv. counter: 49
_abc_registry: {<weakref at 0x0000027AFC3520E0; to 'type' at 0x0000027AFB364650 (TomboList)>}
_abc_cache: set()
_abc_negative_cache: {<weakref at 0x0000027AFC352040; to 'type' at 0x0000027AFB364650 (TomboList)>}
_abc_negative_cache_version: 48
其中的 _abc_registry 的內容和我在 Python 3.4 環境下調用 print(Tombola._abc_registry) 打印的結果是一樣的。
至此，我們大致解決了這個問題。尚有一點不明確的，就是 Python 高版本中是否有其他方式可以在我們自己的代碼中獲取這個 _abc_registry 數據屬性。我目前還沒有找到這個方法。