Luca Lanziani

Thu 29 January 2015


Python - validate command line args

Ok, in the last article I've shown you a tool to write a nice command line interface, but it doesn't help you on validation.

    level = int(args['--log-level'])
except ValueError:
    raise ArgsError('--log-level has to be a valid integer')

Manual validation? There is a much better way to do it, using Schema.

schema is a library for validating Python data structures, such as those obtained from config-files, forms, external services or command-line parsing, converted from JSON/YAML (or something else) to Python data-types.

So we are not bounded to docopt validation, we can actually use this library for many other validation tasks. I'm not going to present the library itself because it's possible to find all the example on the github page.

However I'll leave the docopt validation example, just to show you something and give you an excuse to read more about it ;).


Assume you have this usage string:

Usage: [--count=N] <path> <files>...

That correspond to the following output of docopt:

args = {
    '<files>': ['LICENSE-MIT', ''],
    '<path>': '../',
    '--count': '3'

You can validate it using Schema like this:

from schema import Schema, And, Or, Use
import os
s = Schema({'<files>': [Use(open)],
            '<path>': os.path.exists,
            '--count': Or(None, And(Use(int), lambda n: 0 < n < 5))})
args = s.validate(args)

This will raise an exception in case of some args not respecting the specified rules or it will return the args applying the given operations on them (Use(int) and Use(open)_) otherwise.

In this case it will return the following data:

args = {
    '<files>': [<open file 'LICENSE-MIT', mode 'r' at 0x...>, <open file '', mode 'r' at 0x...>],
    '<path>': '../',
    '--count': 3

Note how '--count' now is no more a string but it has been converted via Use(int).

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