argparse_config / argparse_config.src / README.rst

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', type='store_const', const=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:

[merge]
tool: meld

[commit]
user: Tikitu de Jager <tikitu@logophile.org>

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(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
'meld'

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:

[default]
verbose

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

How does it work?

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

Gotchas

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.)

Hacking

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

TODO

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 things I plan to add:

  • A utility to output a config file, based on a set of commandline arguments.
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