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Default values for argparse commandline args read from a config file.

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The argparse_config utility reads defaults for commandline args from a config file. The cute thing is, it figures out what config options to expect based on your argparse commandline args definition.

Let’s say I’m reimplementing the Mercurial commandline client. I specify the commandline argument processing with argparse, of course:

>>> arg_parser = ArgumentParser('hg')
>>> arg_parser.add_argument('--repository')
>>> sub_parsers = arg_parser.add_subparsers()
>>> merge_parser = sub_parsers.add_parser('merge')
>>> merge_parser.add_argument('--tool')
>>> merge_parser.add_argument('--force', action='store_true', default=False)
>>> commit_parser = sub_parsers.add_parser('commit')
>>> commit_parser.add_argument('--user')
>>> commit_parser.add_argument('--message')

When I go to use this client, though, I have to keep specifying my --user with every commit, and --tool with every merge. That sucks! What I want is to have my client understand a simple config file format:

tool: meld

user: Tikitu de Jager <>

And obviously, as I add more arguments and subcommands to my client, it should allow me to add defaults in the config file without writing more code.

This is what argparse_config gives you. To use it with the mercurial client arg_parser above:

>>> import argparse_config
>>> argparse_config.read_config_file(arg_parser, '/home/tikitu/.my_hg.cfg')

… and that’s it. Calling arg_parser.parse_args() will parse args as usual, but the default values will be taken from the config file, if they are given there:

>>> parsed_args = arg_parser.parse_args(['merge'])
>>> parsed_args.tool

What can I put in the config file?

Under the hood argparse_config uses the standard library ConfigParser. Arguments that aren’t for a subcommand go in the section [default]. The names are munged from the commandline argument, removing leading dashes and converting internal dashes to underscores (e.g. --log-level becomes log_level:).

Flags (i.e. commandline args that take no parameters) are turned on if present in the config, just like the commandline:


is the equivalent of --verbose. Either verbose: or verbose will work, but (watch out!) verbose: a-value doesn’t do anything different to verbose.

Writing a config file from some commandline arguments

Included in the package is a utility to generate a config file following these rules, from a given set of commandline arguments. The easy way to use it is to let it add a command to your args parsing:

>>> config_parser = get_config_parser('/home/tikitu/.my_hg.cfg')
>>> add_config_block_subcommand(arg_parser, sub_parsers, config_parser=config_parser)

and use it on the commandline:

$ config default --repository ssh:// > new_config.rc
$ config commit --username 'Tikitu de Jager <>' >> new_config.rc
$ cat new_config.rc
repository: ssh://
username: Tikitu de Jager <>

You can also use it programatically, if you like. That looks like this:

>>> parsed_args = arg_parser.parse_args(['--repository', '', 'merge'])
>>> print argparse_config.generate_config(arg_parser, parsed_args, section='default', only_non_defaults=True)

Some complications make this less useful than it could be, sadly:

  • If you use subcommands, you can only parse the args for one of them at a time (use the section argument to specify which, or leave it off to get the [default] section).
  • We can’t tell the difference between default values written in code (which should not be added to the config file) and written in a previously-read-in config file (which should). This is why only_non_defaults exists.

How does it work?

By gudgeling about in the private internals of argparse. Yes, that’s not pretty.


Any required arguments that are present in a config file will show as optional, not required, in the --help output. (This is a bug-by-design, due to not having any clever idea about how to do it better.) It may help to tell yourself, “It’s not required on the commandline because I gave it in the config file.” (I will gladly make this dodgy rationalisation disappear if I figure out how to handle required arguments more tidily.)


It’s on BitBucket. Feel free to play. It comes with a handy zc.buildout wrapper too, overkill though that clearly is.


It’s “alpha software” at present; likely to be buggy and lots of stuff ain’t there yet. Check the issues list to stay up to date. Some remaining open issues:

  • How to deal with multi-value args? (The config-file library this is built on doesn’t support them.)
  • The “write me a config file” support is scrappy. Can we do better?

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