为什么在腌制后在Python isinstance中会出现意外行为? [英] Why do I get unexpected behavior in Python isinstance after pickling?

问题描述

抛开使用 isinstance是否有害，我遇到了通过Pickle对对象进行序列化/反序列化后尝试评估isinstance时遇到以下难题:

Putting aside whether the use of isinstance is harmful, I have run into the following conundrum when trying to evaluate isinstance after serializing/deserializing an object via Pickle:

from __future__ import with_statement
import pickle

# Simple class definition
class myclass(object):
    def __init__(self, data):
        self.data = data

# Create an instance of the class
x = myclass(100)

# Pickle the instance to a file
with open("c:\\pickletest.dat", "wb") as f:
    pickle.dump(x, f)

# Replace class with exact same definition
class myclass(object):
    def __init__(self, data):
        self.data = data

# Read an object from the pickled file
with open("c:\\pickletest.dat", "rb") as f:
    x2 = pickle.load(f)

# The class names appear to match
print x.__class__
print x2.__class__

# Uh oh, this fails...(why?)
assert isinstance(x2, x.__class__)

任何人都可以阐明为什么ininstance在这种情况下会失败吗?换句话说，为什么Python认为这些对象属于两个不同的类?当我删除第二个类定义时，isinstance可以正常工作.

Can anyone shed some light on why isinstance would fail in this situation? In other words, why does Python think these objects are of two different classes? When I remove the second class definition, isinstance works fine.

推荐答案

解吸程序的工作方式如下(site-packages/pickle.py):

This is how the unpickler works (site-packages/pickle.py):

def find_class(self, module, name):
    # Subclasses may override this
    __import__(module)
    mod = sys.modules[module]
    klass = getattr(mod, name)
    return klass

查找并实例化一个类.

因此，如果您用相同名称的类替换一个类，则klass = getattr(mod, name)将返回新类，并且该实例属于该新类，因此isinstance将失败.

So of course if you replace a class with an identically named class, the klass = getattr(mod, name) will return the new class, and the instance will be of the new class, and so isinstance will fail.

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