Use of “global” keyword in Python

Question

What I understand from reading the documentation is that Python has a separate namespace for functions, and if I want to use a global variable in that function, I need to us

Answer 1

This is the difference between accessing the name and binding it within a scope.

If you're just looking up a variable to read its value, you've got access to global as well as local scope.

However if you assign to a variable who's name isn't in local scope, you are binding that name into this scope (and if that name also exists as a global, you'll hide that).

If you want to be able to assign to the global name, you need to tell the parser to use the global name rather than bind a new local name - which is what the 'global' keyword does.

Binding anywhere within a block causes the name everywhere in that block to become bound, which can cause some rather odd looking consequences (e.g. UnboundLocalError suddenly appearing in previously working code).

>>> a = 1
>>> def p():
    print(a) # accessing global scope, no binding going on
>>> def q():
    a = 3 # binding a name in local scope - hiding global
    print(a)
>>> def r():
    print(a) # fail - a is bound to local scope, but not assigned yet
    a = 4
>>> p()
1
>>> q()
3
>>> r()
Traceback (most recent call last):
  File "", line 1, in 
    r()
  File "", line 2, in r
    print(a) # fail - a is bound to local scope, but not assigned yet
UnboundLocalError: local variable 'a' referenced before assignment
>>>