Visibility of global variables in imported modules

Question

I\'ve run into a bit of a wall importing modules in a Python script. I\'ll do my best to describe the error, why I run into it, and why I\'m tying this particular approach t

Answer 1

Globals in Python are global to a module, not across all modules. (Many people are confused by this, because in, say, C, a global is the same across all implementation files unless you explicitly make it static.)

There are different ways to solve this, depending on your actual use case.

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Before even going down this path, ask yourself whether this really needs to be global. Maybe you really want a class, with f as an instance method, rather than just a free function? Then you could do something like this:

import module1
thingy1 = module1.Thingy(a=3)
thingy1.f()

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If you really do want a global, but it's just there to be used by module1, set it in that module.

import module1
module1.a=3
module1.f()

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On the other hand, if a is shared by a whole lot of modules, put it somewhere else, and have everyone import it:

import shared_stuff
import module1
shared_stuff.a = 3
module1.f()

… and, in module1.py:

import shared_stuff
def f():
    print shared_stuff.a

Don't use a from import unless the variable is intended to be a constant. from shared_stuff import a would create a new a variable initialized to whatever shared_stuff.a referred to at the time of the import, and this new a variable would not be affected by assignments to shared_stuff.a.

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Or, in the rare case that you really do need it to be truly global everywhere, like a builtin, add it to the builtin module. The exact details differ between Python 2.x and 3.x. In 3.x, it works like this:

import builtins
import module1
builtins.a = 3
module1.f()