Python：在其中的模块和类之间共享全局变量 [英] Python: Sharing global variables between modules and classes therein

问题描述

我知道可以在Python中跨模块共享一个全局变量。但是，我想知道这是可能的程度，为什么。例如，

global_mod.py

  x =无$ b b 
 
 
 $ mid_access_mod.py c $ c> from global_mod import * 
 
 class delta：
 def __init __（self）：
 print x 
  

$ b bot_modif_mod.py

  import mid_access_mod 
 import global_mod 
 
 class mew：
 def __init __（self）：
 global_mod.x = 5 
 
 def main（）：
m = mew（） 
d = mid_access_mod.delta（）
  

即使所有模块共享全局变量x。为什么会这样？看起来像x在mid_access_mod.py被mew（）分配到bot_modif_mod.py之前被评估过。 这会发生因为你正在使用不可变的值（ints和None），并且导入变量就像是按值传递东西，而不是通过引用传递东西。

如果您制作了global_mod.xa列表，并操纵它的第一个元素，它就会像你期望的那样工作。

当你从global_mod import x 执行时，您正在模块中创建名称 x ，其值与 x 在 global_mod中的值相同。对于像函数和类这样的东西，这可以像预期的那样工作，因为人们（通常）不会在以后重新分配这些名称。

正如Alex指出的那样，如果您使用 import global_mod ，然后使用 global_mod.x ，您将避免该问题。您在模块中定义的名称将是 global_mod ，它总是指您想要的模块，然后使用属性访问获取 x 会为您带来最新的 x 。

I know that it's possible to share a global variable across modules in Python. However, I would like to know the extent to which this is possible and why. For example,

global_mod.py

x = None

mid_access_mod.py

from global_mod import *

class delta:
    def __init__(self):
        print x

bot_modif_mod.py

import mid_access_mod
import global_mod

class mew:
    def __init__(self):
        global_mod.x = 5

def main():
    m = mew()
    d = mid_access_mod.delta()

This prints None, even though all the modules are sharing the global variable x. Why is this the case? It seems like x is evaluated at mid_access_mod.py before it is assigned in bot_modif_mod.py by mew().

解决方案

This happens because you are using immutable values (ints and None), and importing variables is like passing things by value, not passing things by reference.

If you made global_mod.x a list, and manipulated its first element, it would work as you expect.

When you do from global_mod import x, you are creating a name x in your module with the same value as x has in global_mod. For things like functions and classes, this works as you would expect, because people (generally) don't re-assign to those names later.

As Alex points out, if you use import global_mod, and then global_mod.x, you will avoid the problem. The name you define in your module will be global_mod, which always refers to the module you want, and then using attribute access to get at x will get you the latest value of x.

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