A Python 2.7 programming tutorial

If you want to make some of the arguments to your function optional, you must supply a default value. In the argument list, this looks like “name=value”.

Here's an example of a function with one argument that has a default value. If you call it with no arguments, the name mood has the string value 'bleah' inside the function. If you call it with an argument, the name mood has the value you supply.

>>> def report(mood='bleah'):
...     print "My mood today is", mood
... 
>>> report()
My mood today is bleah
>>> report('hyper')
My mood today is hyper
>>> 

If your function has multiple arguments, and the caller supplies multiple parameters, here is how they are matched up:

Here are some examples showing how this works.

>>> def f(a, b="green", c=3.5):
...     print a, b, c
... 
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: f() takes at least 1 argument (0 given)
>>> f(47)
47 green 3.5
>>> f(47, 48)
47 48 3.5
>>> f(47, 48, 49)
47 48 49
>>> f(47, 48, 49, 50)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: f() takes at most 3 arguments (4 given)
>>> 

Here is another feature: the caller of a function can supply what are called keyword parameters of the form “name=value”. If the function has an argument with a matching keyword, that argument will be set to value.

Here are some examples of calling a function with keyword parameters.

>>> def g(p0, p1, k0="K-0", k1="K-1"):
...     print p0, p1, k0, k1
... 
>>> g(33,44)
33 44 K-0 K-1
>>> g(33,44,"K-9","beep")
33 44 K-9 beep
>>> g(55,66,k1="whirr")
55 66 K-0 whirr
>>> g(7,8,k0="click",k1="clank")
7 8 click clank
>>>