Functions

Functions are defined using the def statement.
They are called using the function name followed by parentheses, with any arguments inside the parentheses. 

>>> def double(x):
...     print x * 2
...
>>> double(3)
6

A value can be returned from a function by the return statement. 

>>> def factorial(n):
...     if n > 0:
...             return n * factorial(n-1)
...     else:
...             return 1
...
>>> x = factorial(5)
>>> print x
120

Variable scoping

By default, variables that are assigned to in a function only exist within that function.
i.e. they have local scope.

The global statement is used to declare that a variable should exist in the global scope.
This will make it visible outside of the function, after the function has been called. 

>>> def foo(a):
...     y = a
...     print y
...
>>> foo(7)
7
>>> print y
Traceback (most recent call last):
  File "", line 1, in ?
NameError: name 'y' is not defined



>>> def foo(a):
...     global y
...     y = a
...     print y
...
>>> foo(12)
12
>>> print y
12

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