为什么在酸洗后Python isinstance中会出现意外行为?

搁置是否使用 isinstance is harmful

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__)

有人能解释一下为什么iInstance会在这种情况下失败吗?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 很好。