为什么在酸洗之后我会在Python中出现意外行为？

Question

暂时不管使用isinstance是否有害,在尝试通过Pickle序列化/反序列化对象后尝试评估isinstance时,我遇到了以下难题:

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__)

任何人都能解释为什么在这种情况下实例会失败吗？换句话说,为什么Python认为这些对象是两个不同的类？当我删除第二个类定义,isinstance工作正常.

Answer 1

答案显而易见，因为它不是同一类。

它是一个相似的类，但不一样。

class myclass(object):
    pass

x = myclass()

class myclass(object):
    pass

y = myclass()


assert id(x.__class__) == id(y.__class__) # Will fail, not the same object

x.__class__.foo = "bar"

assert y.__class__.foo == "bar" # will raise AttributeError

Answer 2

这就是unpickler的工作原理(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.

所以当然如果你用一个同名的类替换一个类,klass = getattr(mod, name)它将返回新类,而实例将属于新类,因此isinstance将失败.