Python 中模块间全局变量的使用上的注意

最近用Python写代码，需要用到模块间的全局变量。

网上四处搜索，发现普遍做法是把全局变量放到一个独立的模块中，使用时，导入此全局变量模块即可。

但是在实际使用过程中发现了些小问题：在使用如下代码导入全局变量模块时，各个模块获取到的全局变量都是初始值。

from module import global_var

但是如果使用「模块名.全局变量名」来访问时，却又是正常的：

import module

print module.global_var

为了弄清其中的原委，我写了个测试程序来仔细查看其中的细节：

1. import module

global_var.py

GLOBAL_VAR = [1, 2, 3]

m1.py

import global_var
import m2

print 'm1: before appending:          ', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR
global_var.GLOBAL_VAR.append('m1')
print 'm1: after appending:           ', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR

print 'm1: before calling m2.append():', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR
m2.append()
print 'm1: after calling m2.append(): ', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR

print '-----------------'

print 'm1: before assigning:          ', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR
global_var.GLOBAL_VAR = ['m1']
print 'm1: after assigning:           ', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR

print 'm1: before calling m2.assign():', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR
m2.assign()
print 'm1: after calling m2.assign(): ', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR

m2.py

import global_var

def append():
    print 'm2: before assiging:           ', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR
    global_var.GLOBAL_VAR.append('m2')
    print 'm2: after assiging:            ', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR

def assign():
    print 'm2: before assiging:           ', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR
    global_var.GLOBAL_VAR = ['m2']
    print 'm2: after assiging:            ', id(global_var.GLOBAL_VAR), global_var.GLOBAL_VAR

运行结果：

Mac-pastgift:pytest pastgift$ python global_test_import/m1.py
m1: before appending:           4457308312 [1, 2, 3]
m1: after appending:            4457308312 [1, 2, 3, 'm1']
m1: before calling m2.append(): 4457308312 [1, 2, 3, 'm1']
m2: before assiging:            4457308312 [1, 2, 3, 'm1']
m2: after assiging:             4457308312 [1, 2, 3, 'm1', 'm2']
m1: after calling m2.append():  4457308312 [1, 2, 3, 'm1', 'm2']
-----------------
m1: before assigning:           4457308312 [1, 2, 3, 'm1', 'm2']
m1: after assigning:            4457444848 ['m1']
m1: before calling m2.assign(): 4457444848 ['m1']
m2: before assiging:            4457444848 ['m1']
m2: after assiging:             4457308312 ['m2']
m1: after calling m2.assign():  4457308312 ['m2']

在这种方式下，如果是改变对象本身的操作（append），各个模块内部的操作都是针对于同一个对象。

而赋值操作，虽然全局变量所指向的对象改变了，但是全局变量本身依然在各个模块内部能够正确访问，这正是我所希望的效果。

2. from module import GLOBAL_VAR

global_var.py

GLOBAL_VAR = [1, 2, 3]

m1.py

from global_var import GLOBAL_VAR
import m2

print 'm1: before appending:          ', id(GLOBAL_VAR), GLOBAL_VAR
GLOBAL_VAR.append('m1')
print 'm1: after appending:           ', id(GLOBAL_VAR), GLOBAL_VAR

print 'm1: before calling m2.append():', id(GLOBAL_VAR), GLOBAL_VAR
m2.append()
print 'm1: after calling m2.append(): ', id(GLOBAL_VAR), GLOBAL_VAR

print '-----------------'

print 'm1: before assigning:          ', id(GLOBAL_VAR), GLOBAL_VAR
GLOBAL_VAR = ['m1']
print 'm1: after assigning:           ', id(GLOBAL_VAR), GLOBAL_VAR

print 'm1: before calling m2.assign():', id(GLOBAL_VAR), GLOBAL_VAR
m2.assign()
print 'm1: after calling m2.assign(): ', id(GLOBAL_VAR), GLOBAL_VAR

m2.py

from global_var import GLOBAL_VAR

def append():
    global GLOBAL_VAR

    print 'm2: before assiging:           ', id(GLOBAL_VAR), GLOBAL_VAR
    GLOBAL_VAR.append('m2')
    print 'm2: after assiging:            ', id(GLOBAL_VAR), GLOBAL_VAR

def assign():
    global GLOBAL_VAR

    print 'm2: before assiging:           ', id(GLOBAL_VAR), GLOBAL_VAR
    GLOBAL_VAR = ['m2']
    print 'm2: after assiging:            ', id(GLOBAL_VAR), GLOBAL_VAR

运行结果：

Mac-pastgift:pytest pastgift$ python global_test_from_import/m1.py
m1: before appending:           4539998360 [1, 2, 3]
m1: after appending:            4539998360 [1, 2, 3, 'm1']
m1: before calling m2.append(): 4539998360 [1, 2, 3, 'm1']
m2: before assiging:            4539998360 [1, 2, 3, 'm1']
m2: after assiging:             4539998360 [1, 2, 3, 'm1', 'm2']
m1: after calling m2.append():  4539998360 [1, 2, 3, 'm1', 'm2']
-----------------
m1: before assigning:            [1, 2, 3, 'm1', 'm2']
m1: after assigning:            4540135112 ['m1']
m1: before calling m2.assign(): 4540135112 ['m1']
m2: before assiging:             [1, 2, 3, 'm1', 'm2']
m2: after assiging:             4540135040 ['m2']
m1: after calling m2.assign():  4540135112 ['m1']

这次，运行结果和上次略有不同。

改变对象本身的操作（append）和之前的例子一样，全局变量始终指向相同的对象。

但是赋值操作就比较奇怪了。注意标红部分，每个模块第一次获取到的这个「全局变量」都是相同id的对象。即使这个「全局变量」在其他模块中已经被重新赋值，但在本模块中依然指向的是最原始的id。

显然，这个算不上是「全局变量」了。