Why Clize was made

Clize started from the idea that other argument parsers were too complicated to use. Even for a small script, one would have to use a fairly odd interface to first generate a « parser » object with the correct behavior then use it.

import argparse

parser = argparse.ArgumentParser()
parser.add_argument("name", nargs='?',
                    help="The person to greet")
parser.add_argument("repeat", nargs='?', type=int, default=1,
                    help="How many times should the person be greeted")
parser.add_argument("--no-capitalize", action='store_const', const=True)

args = parser.parse_args()

if args.name is None:
    greeting = 'Hello world!'
    name = args.name if args.no_capitalize else args.name.title()
    greeting = 'Hello {0}!'.format(name)

for _ in range(args.repeat):

This doesn’t really feel like what Python should be about. It’s too much for such a simple CLI.

A much simpler alternative could be to use argument unpacking:

import sys.argv

 def main(script, name=None, repeat='1'):
     if name is None:
         greeting = 'Hello world!'
         greeting = 'Hello {0}!'.format(name.title())

     for _ in range(int(repeat)):


This makes better use of Python concepts: a function bundles a series of statements with a list of parameters, and that bundle is now accessible from the command-line.

However, we lose several features in this process: Our simpler version can’t process named arguments like --no-capitalize, there is no --help function of any sort, and all errors just come up as tracebacks, which would be confusing for the uninitiated.

Those shortcomings are not inherent to bundling behavior and parameters into functions. Functions can have keyword-only parameters (and this can be backported to Python 2.x), and those parameter lists can be examined at run time. Specific errors can be reformatted, and so forth.

Clize was made to address these shortcomings while expanding on this idea that command-line parameters are analogous to those of a function call in Python.

The following table summarizes a few direct translations Clize makes:

Python construct

Command-line equivalent



List of functions

Multiple commands


Source for the --help output


Transformation of a command

Positional parameter

Positional parameter

Keyword-only parameter

Named parameter (like --one)

Parameter with a default value

Optional parameter

Parameter with False as default

Flag (True if present, False otherwise)

Some concepts fall outside these straightforward relationships, but in all cases your part of the command-line interface remains a normal function. You can call that function normally, have another command from your CLI use it, or test it like any other function.

For when Python constructs aren’t enough, Clize uses parameter annotations, a yet mostly unexplored feature of Python 3. For instance, you can specify value converters for the received arguments, or replace how a parameter is implemented completely.

Even though its basic purpose could be called magicky, Clize attempts to limit magic in the sense that anything Clize’s own parameters do can also be done in custom parameters. For instance, --help in a command-line will trigger the displaying of the help screen, even if there were errors beforehand. You might never need to do this, but the option is there if and when you ever need this. argparse for instance does not let you do this.

With Clize, you start simple but you remain flexible throughout, with options for refactoring and extending your command-line interface.

“Why not use an existing parser instead of making your own?”

Argument parsing is a rather common need for Python programs. As such, there are many argument parsers in existence, including no less than three in the standard library alone!

The general answer is that they are different. The fact that there are so many different parsers available shows that argument parsing APIs are far from being a “solved problem”. Clize offers a very different approach from those of argparse, Click or docopt. Each of these always have you write a specification for your CLI.

Clize comes with less batteries included than the above. It focuses on providing just the behavior that corresponds with Python parameters, along with just a few extras. Clize can afford to do this because unlike these, Clize can be extended to accommodate custom needs. Every parameter kind can be implemented by external code and made usable the same way as clize.parameters.multi or clize.parameters.argument_decorator.

“Why not create a thin wrapper around argparse?”

Back during Clize’s first release, argparse’s parser would have been sufficient for what Clize proposed, though I wasn’t really interested in dealing with it at the time. With Clize 3’s extensible parser, replacing it with argparse would be a loss in flexibility, in parameter capabilities and help message formatting.