Python全局变量未按预期工作

Question

I'm just starting out with Python and experimenting with different solutions. I was working with global variables and I ran into something but don't know why it's doing what it's doing.

To start, I have two modules: test1 and test2. test1 is as follows:

import test2
num = 0
def start():
    global num
    num = num + 5
    print 'Starting:'
    print num
    test2.add()
    print 'Step 1:'
    print num
    test2.add()
    print 'Step 2:'
    print num

And test2 is this:

import test1
def add():
    test1.num = test1.num + 20

When I run test1.start() the output is:

Starting:
5
Step 1:
25
Step 2:
45

Why doesn't test2 need the global declaration to modify the variable in test1? Line 5 in test1 requires it at line 4, but if I remove both it still works (0, 20,40). I'm just trying to figure out why it's not working as I expected it to.

Thanks.

Answer 1

The global declaration is not for modifying the name, it's for rebinding it. Since you are accessing the name via its module what you are doing is modifying the module .

Answer 2

From test2's perspective, test1.num is a variable belonging to the module test1.

The global keyword only specifics that the scoping for that variable is module-level (ie not local).