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