模块重新加载的isinstance行为 [英] isinstance behavior with module reload
问题描述
给出以下两个.py文件:
Given the following two .py files:
aclass.py
aclass.py
class A(object):
pass
main.py
def importer(klass):
"""
Used to import classes from there python qalname
"""
import_ = lambda m, k: getattr(__import__(m, fromlist=k), k)
klass = klass.split('.')
module = '.'.join(klass[:-1])
klass = klass[-1]
return import_(module, klass)
from aclass import A
import_A = importer('aclass.A')
print isinstance(A(), import_A) # Expected to be true
print isinstance(import_A(), A) # Expected to be true
在这个阶段,一切正常(我的程序打印 True\\\
)
True
但是如果我修改 importer
方法来强制重新加载,即:
At this stage, everything works fine (my program prints True\nTrue
)
But if I modify the importer
method to enforce a reload, ie:
这一行:
import_ = lambda m, k: getattr(__import__(m, fromlist=k), k)
被替换为:
import_ = lambda m, k: getattr(reload(__import__(m, fromlist=k)), k)
我的程序返回
False
False
我不明白这种行为。
推荐答案
重新加载模块意味着重新执行其内容,在本例中为 A类(对象):传递
。所以它创造了另一个不同的类。它的行为与以下相同:
Reloading a module means re-executing its content, in this case class A(object): pass
. So it creates another different class. It's the same behavior as:
class A(object):
pass
a = A()
class A(object): # a different class
pass
print isinstance(a, A) # False
这应该足以解释为什么裸 reload()
通常是个坏主意。我相信其他人可以指出实现更复杂的重新加载程序的框架,例如修补旧类被认为等于新类。
This should be enough to explain why a bare reload()
is usually a bad idea. I'm sure others could point to frameworks that implement more sophisticated reloading procedures, e.g. patching the old class to be considered equal to the new one.
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