How does argparse's add_argument take variable length arguments before keyword arguments?

Question

In python2.7, the argparse module has an add_argument method which can take a variable number of unnamed arguments before its keyword arguments as shown below:

parser = argparse.ArgumentParser(description='D')
parser.add_argument('-a', '-b', ... '-n', action='store', ... <other keyword args>)

As far as I know, function definitions as follow don't work:

def fxn(var_args*, action, otherstuff):
def fxn(action, otherstuff, var_args*): # results in conflict on action

What is the proper means to emulate the add_argument behaviour?

Answer 1

Python's argument definition order is...

1. Required and/or default-value arguments (if any)
2. variable-length positional arguments placeholder (*<name> if desired)
3. keyword arguments placeholder (**<name> if desired)

The positional arguments placeholder gets a list, the keyword arguments placeholder gets a dict.

add_arguments simply looks for keys in the dict of keyword arguments, rather than spelling out all of the possible arguments in the declaration. Something along the lines of...

def add_arguments(*posargs, **kwargs):
    if 'action' in kwargs:
        # do something

Answer 2

You can pass arbitrary number of arguments to a function.

Here is an example

def f(x, *args, **kwargs):
    print x
    for arg in args:
        print arg
    for key, value in kwargs:
        print key + ': ' + value

Reading this will help: http://docs.python.org/tutorial/controlflow.html#keyword-arguments.