在python中跨类更改全局变量

Question

I am trying to make a change to a global variable across classes. Here is my code:

file main.py

import class2

class first:
    def changeA(self):
        global a
        print a
        a += 2
        print a

test0 = first()
test1 = class2.second()
test2 = first()
test3 = class2.second()
test0.changeA()
test1.changeA()
test2.changeA()
test3.changeA()

file class2.py

a = 1

class second:
    def changeA(self):
        global a
        print a
        a += 1
        print a

But it returns an error: global name 'a' is not defined. Is there any proper way to access and change a global variable across files in python? Thanks in advance

Answer 1

Global variables don't exist in python.

The global statement is really a misnomed statement. What it means is that the variable is a module 's variable. There is no such a thing as a global namespace in python.

If you want to modify a variable from multiple modules you must set it as an attribute:

import module

module.variable = value

Doing a simple assignment will simply create or modify a module's variable. The code

from module import variable

variable = value

simply shadows the value of variable imported from module creating a new binding with that identifier but module 's variable value will not be changed.

In summary: no there is no way to achieve exactly what you want (although what you want would be a bad practice anyway, and you should try to use a different solution).

Answer 2

global variables are evil: avoid them!

It is much better to use a 'static' (in C++ terms) member variable, such as:

from class2 import second
class first:
    def changeA(self):
        print second.a
        second.a += 2
        print second.a

And:

class second:
    a = 1
    def changeA(self):
        print second.a
        second.a += 2
        print second.a