Entry

How does the statement 'global' work?

Mar 31st, 2009 11:34
Scott Mandarich, unknown unknown, Remco Gerlich, Thomas Wouters

Answer1:

Inside functions, if you assign to a variable, it is assumed to be 
local 
to the function. If you want to assign to a global (module namespace) 
variable, you'll have to tell Python that it's a global first. 

ie,

x = 4
def spam():
  x = 5
spam()

Doesn't change the module's x. But

x = 4
def spam()
  global x
  x = 5
spam()

does.

If, inside a function, you only use the variable and never assign to 
it, 
it can't be a local variable, so Python assumes you mean a global.

This doesn't only hold for functions, but classes and methods too.

Answer2:

Python has two namespaces it uses for name lookups: the 'local' 
namespace and the 'global' namespace (you could call it the 'search 
path'.) The local namespace is the function or class you are in, the 
global namespace is the namespace of the module. If you are not in a 
function or class, the local namespace is the global namespace.

However, *assigning* to a variable does not use this search path ! 
Instead, if you assign, you always assign in the local namespace. So, 
for instance:

X = None

def FillX(x):
        X = x

would not work, because the X you assign to, is the *local* X, not the
global one. This wouldn't even generate an error, by the way. Even more
confusing is it if you read from and assign to 'X' in the same 
function:

X = None

def FillX(x):
        if not X:
                X = x

In Python 1.5.2, this generates a NameError:

Traceback (innermost last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 2, in FillX
NameError: X

and in Python 1.6, an UnboundLocalError:

Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in FillX
UnboundLocalError: X

It generates this strange error because the 'X' name exists in the 
local
namespace, but at the time of the test on X, it isn't filled in yet. 
The
interpreter knows it should be there, but can't find it at the time of
execution.

And this is what the 'global' keyword is for. It says 'this name is 
global, *even if* it gets assigned to. You only need it when you are 
assigning to globals, not when you are mutating them (appending to 
lists, adding to dictionaries, etc.)














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