(python2.1-tut.info)Default Argument Values


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


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