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.