4.7.1 Default Argument Values

The most useful form is to specify a default value for one or more arguments. This creates a function that can be called with fewer arguments than it is defined, e.g.

def ask_ok(prompt, retries=4, complaint='Yes or no, please!'):
    while 1:
        ok = raw_input(prompt)
        if ok in ('y', 'ye', 'yes'): return 1
        if ok in ('n', 'no', 'nop', 'nope'): return 0
        retries = retries - 1
        if retries < 0: raise IOError, 'refusenik user'
        print complaint

This function can be called either like this: ask_ok('Do you really want to quit?') or like this: ask_ok('OK to overwrite the file?', 2).

The default values are evaluated at the point of function definition in the defining scope, so that e.g.

i = 5
def f(arg = i): print arg
i = 6
f()

will print 5.

Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list or dictionary. For example, the following function accumulates the arguments passed to it on subsequent calls:

def f(a, l = []):
    l.append(a)
    return l
print f(1)
print f(2)
print f(3)

This will print

[1]
[1, 2]
[1, 2, 3]

If you don't want the default to be shared between subsequent calls, you can write the function like this instead:

def f(a, l = None):
    if l is None:
        l = []
    l.append(a)
    return l