Using global variables in python in a function

In the first case the value remains same in all the functions.

def set_globvar_to_one():
    global globvar
    globvar = 1
    print globvar

def print_globvar():
    print globvar     # No need for global declaration to read value of globvar

set_globvar_to_one()
print_globvar()

The output will be 1 and 1.

But 2nd case

def set_globvar_to_one():
    globvar = 1
    print globvar

def print_globvar():
    print globvar     # No need for global declaration to read value of globvar

set_globvar_to_one()
print_globvar()

Here you get error saying globvar is not defiend, coz there is no local or global variable named globavar

That would make it ambiguous. Look at this,

a = 0
def foo():
    a = 10 # *

Look at the line marked *. Are you assigning to the global a or is this a local variable a with a value 10. The interpreter has no way to knowing what you meant. Thus, the need for global.

a = 0
def foo():
    a = 10 # local variable a

def bar():
    global a # will be modifying global variable a in this function
    a = 10 # assign 10 to global variable 

In your first example "a" is not available before foo() is called. Once it is defined in the function it available outside of the function.

In the second example it is available before foo() is called butchanges to it inside the function do not affect the global version .

This example should demonstrate the difference between the two examples

#!/usr/bin/python

def foo():
    global a
    a = 10
    print a

try:
 print a
except Exception,e:
 print e

foo()
print a

print "-"*70

b = 0
def bar():
    b = 10
    print b

print b
bar()
print b

Output:

global name 'a' is not defined
10
10
----------------------------------------------------------------------
0
10
0

If you declare it as a global, you don't have to return it. If you don't use global, you have the function to return a. Without global it be like this:

In [7]: def foo():
   ...:     a = 10
   ...:     return a
   ...: 

In [9]: a = foo()

In [10]: a
Out[10]: 10

With global:

In [11]: a = 0

In [12]: def foo2():
   ....:     global a
   ....:     a = 10
   ....:     

In [13]: a
Out[13]: 0

In [14]: foo2()

In [15]: a
Out[15]: 10

It is you who decide if you want your function to return or to use global. It can simplify you code, it can be useful for function that calculates some constants that you don't want to call it each time. But if use it for single procedure, prefer "return" method.

if you are using the same variable inside the loop then there will be a competition between the global and local variable . so if you want to use the value of the global variable in the loop you have to mention the 'global' keyword