Your question is worded... strangely. Are you having trouble with setting default arguments in a function definition?
>>> def f(arg1="hello", arg2="goodbye"):
print "arg1 is", arg1
print "arg2 is", arg2
>>> f()
arg1 is hello
arg2 is goodbye
>>> f(arg2="two")
arg1 is hello
arg2 is two
>>> f(1,2)
arg1 is 1
arg2 is 2
>>> f(arg2="foo", arg1="bar")
arg1 is bar
arg2 is foo
If that wasn't what you were looking for, did you want to prompt the user for a missing argument?
>>> def g(arg=None):
if arg is None:
arg = raw_input("What is the argument?")
print "The argument was", arg
>>> g(123)
The argument was 123
>>> g()
What is the argument? foo bar
The argument was foo bar
Using a sentinel value of None for a missing argument is the idiomatic way in Python to detect a missing argument and execute another function.