Python Import and Global Variables

Question

Suppose I have a file:

# main.py

import foo

my_global = "global variable"
foo.print_global()

and another:

# foo.py

def print_global():
    global my_global
    print(my_global)

Why do I get NameError: name 'my_global' is not defined when I run main.py please? How can I make my_global available to foo.py ? Is it a bad idea to try - maybe I should always pass values as function arguments in this kind of situation?

Answer 1

Python doesn't have process-wide globals, only module-level globals. foo.print_global looks at foo.my_global , not main.my_global , which is what you set. That is, the scope that print_global uses for global variables is determined when print_global is defined , not when it is called .

This would do what you expect.

import foo

foo.my_global = "global variable"
foo.print_global()

Note that foo.py should not rely on someone else creating its global variable(s) before calling print_global ; at the very least, foo.py should initialize my_global to None , if not some other default value.

Also note that the solution above is not the same as

from foo import my_global

my_global = "..."

This creates a new global name in the current module, which is initialized using the current value of foo.my_global . The subsequent assignment changes the name of the "local" global variable, not foo.my_global .