Question

我创建了空的抽象类AbstractStorage并从中继承了Storage类：

import abc
import pymongo as mongo

host = mongo.MongoClient()

print(host.alive()) # True

class AbstractStorage(metaclass=abc.ABCMeta):
    pass

class Storage(AbstractStorage):
    dbh = host
    def __init__(self):
        print('__init__')

Storage()

我预计输出为

True
__init__

但是，我得到的是

True
Traceback (most recent call last):
  File "/home/vaultah/run.py", line 16, in <module>
    Storage()
TypeError: Can't instantiate abstract class Storage with abstract methods dbh

如果我删除metaclass=abc.ABCMeta（以便AbstractStorage成为普通类）和/或我将dbh设置为其他值，则问题（显然）会消失。

这里发生了什么？

Answer 1

这对ABCs来说确实不是问题，但这是PyMongo的一个问题。有一个问题here。看起来pymongo会覆盖__getattr__来返回某种类型的数据库类。这意味着host.__isabstractmethod__返回一个Database对象，在布尔上下文中为true。这导致ABCMeta认为host是一种抽象方法：

>>> bool(host.__isabstractmethod__)
True

问题报告中描述的解决方法是在您的对象上手动设置host.__isabstractmethod__ = False。关于这个问题的最后评论表明已经为pymongo 3.0提供了修复。

Answer 2

短版

mongo.MongoClient返回一个似乎是（是？）抽象方法的对象，然后将其分配给dbh中的Storage字段。这使得Storage成为一个抽象类，因此实例化它会引发TypeError。

请注意，我没有pymongo，因此我无法告诉您有关MongoClient的更多信息，而不是ABCMeta如何对待它。

长版

ABCMeta.__new__方法查看它正在创建的新类的每个字段。任何本身具有True（或＆＃34;类似于＆＃34;）__isabstractmethod__字段的字段都被视为抽象方法。如果一个类具有任何非重写的抽象方法，则整个类被认为是抽象的，因此任何实例化它的尝试都是错误的。

来自早期版本的标准库abc.py：

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)}
    # ...
    cls.__abstractmethods__ = frozenset(abstracts)
    # ...

abc.ABCMeta类文档中未提及此问题，但在@abc.abstractmethod装饰器下方略低一点：

为了正确地与抽象基类机制进行互操作，描述符必须使用__isabstractmethod__将自身标识为抽象。通常，如果用于组成描述符的任何方法都是抽象的，则此属性应为True。

示例

我创造了一个虚假的＆＃34;抽象的＆＃34;具有__isabstractmethod__属性的类，以及AbstractStorage的两个所谓具体子类。您会看到一个产生您正在获得的确切错误：

#!/usr/bin/env python3 import abc # I don't have pymongo, so I have to fake it. See CounterfeitAbstractMethod. #import pymongo as mongo class CounterfeitAbstractMethod(): """ This class appears to be an abstract method to the abc.ABCMeta.__new__ method. Normally, finding an abstract method in a class's namespace means that class is also abstract, so instantiating that class is an error. If a class derived from abc.ABCMeta has an instance of CounterfeitAbstractMethod as a value anywhere in its namespace dictionary, any attempt to instantiate that class will raise a TypeError: Can't instantiate abstract class <classname> with abstract method <fieldname>. """ __isabstractmethod__ = True class AbstractStorage(metaclass=abc.ABCMeta): def __init__(self): """ Do-nothing initializer that prints the name of the (sub)class being initialized. """ print(self.__class__.__name__ + ".__init__ executing.") return class ConcreteStorage(AbstractStorage): """ A concrete class that also _appears_ concrete to abc.ABCMeta. This class can be instantiated normally. """ whatever = "Anything that doesn't appear to be an abstract method will do." class BogusStorage(AbstractStorage): """ This is (supposedly) a concrete class, but its whatever field appears to be an abstract method, making this whole class abstract --- abc.ABCMeta will refuse to construct any this class. """ #whatever = mongo.MongoClient('localhost', 27017) whatever = CounterfeitAbstractMethod() def main(): """ Print details of the ConcreteStorage and BogusStorage classes. """ for cls in ConcreteStorage, BogusStorage: print(cls.__name__ + ":") print(" whatever field: " + str(cls.whatever)) print(" abstract methods: " + str(cls.__abstractmethods__)) print(" Instantiating...") print(" ", end="") # KABOOM! Instantiating BogusStorage will raise a TypeError, # because it appears to be an _abstract_ class. instance = cls() print(" instance: " + str(instance)) print() return if "__main__" == __name__: main()

运行它会产生：

$ ./storage.py ConcreteStorage: whatever field: Anything that doesn't appear to be an abstract method will do. abstract methods: frozenset() Instantiating... ConcreteStorage.__init__ executing. instance: <__main__.ConcreteStorage object at 0x253afd0> BogusStorage: whatever field: <__main__.CounterfeitAbstractMethod object at 0x253ad50> abstract methods: frozenset({'whatever'}) Instantiating... Traceback (most recent call last): File "./storage.py", line 75, in <module> main() File "./storage.py", line 68, in main instance = cls() TypeError: Can't instantiate abstract class BogusStorage with abstract methods whatever

抽象类和PyMongo;无法实例化抽象类

2 个答案: