A Comprehensive Introduction to Python Programming and ... - MSDL

1.2 Language Features 13 / 75
1.2.6 Function Definitions and Function Calls
Function in Python are declared using the def keyword. The basic syntax is as follows:
def func name(arg1, arg2, . . ., argN):
statements
Listing 14: Functions
Since the language is untyped, no function definition has a return type, and parameters
are simply listed. If a function is to return a value, the keyword return can be
used to return it. The same keyword used without a value will stop the excution of the
function. This also the convention that C and Java use.
Python also supports some interesting features relative to function calls. The first of
these features is that arguments can be passed and filled in two distinct ways: the traditional
(ie C and Java) way, which is by position. Alternatively, Python supports filling
in arguments by name. When arguments are provided by name, then their placement in
the argument list does not matter, since the name has precedence over the place in the
list. To make this concept clearer, here are two examples illustrating the two methods
previously described:
Listing 15: Arguments Example 1 - filled in by position
>> def foo(w, x, y, z):
return (w, x, y, z)
>> foo(1, 2, 3, 4)
(1, 2, 3, 4)
>> 5
Listing 16: Arguments Example 2 - filled in by name
>> def foo(w, x, y, z):
return (w, x, y, z)
>> foo(x = 2, z = 4, w = 1, y = 3)
(1, 2, 3, 4)
>> 5
It is also possible to combine the two methods in the same function call, provided
that all arguments following the first one that is passed by name are also passed by
name. The arguments before the first one that is passed by name will all be passed by
position.
Note that multiple definitions of a single argument will raise an exception. An
exception is also raised if not all arguments receive a value, the arguments that are
ignored have been assigned a default value, as explained next.