抽象类和PyMongo;无法实例化抽象类 [英] Abstract classes and PyMongo; can't instantiate abstract class

问题描述

我创建了一个空的抽象类 AbstractStorage ，并从其中继承了 Storage 类：

I created the empty abstract class AbstractStorage and inherited the Storage class from it:

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()

我期望输出为

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 （显然， AbstractStorage 普通类）和/或（如果我将 dbh 设置为其他值）。

The problem (apparently) goes away if I remove metaclass=abc.ABCMeta (so that AbstractStorage becomes an ordinary class) and/or if I set dbh to some other value.

这是怎么回事？

推荐答案

ABC的问题，PyMongo的问题。 此处，有一个问题。似乎pymongo会覆盖 __ getattr __ 以返回某种数据库类。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相信主机是一种抽象方法：

This isn't really a problem with ABCs, it's a problem with PyMongo. There is an issue about it here. It seems that pymongo overrides __getattr__ to return some sort of database class. This means that host.__isabstractmethod__ returns a Database object, which is true in a boolean context. This cause ABCMeta to believe that host is an abstract method:

>>> bool(host.__isabstractmethod__)
True

问题报告中描述的解决方法是手动在对象上设置 host .__ isabstractmethod__ = False 。关于这个问题的最后评论表明，已经针对pymongo 3.0进行了修复。

The workaround described in the issue report is to manually set host.__isabstractmethod__ = False on your object. The last comment on the issue suggests a fix has been put in for pymongo 3.0.

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