argparse on demand imports for types, choices etc

Question

I have quite a big program which has a CLI interaction based on argparse, with several sub parsers. The list of supported choices for the subparsers arguments a

Answer 1

Here's a quick and dirty example of a 'lazy' choices. In this case choices are a range of integers. I think a case that requires expensive DB lookups could implemented in a similar fashion.

# argparse with lazy choices

class LazyChoice(object):
    # large range
    def __init__(self, argmax):
        self.argmax=argmax
    def __contains__(self, item):
        # a 'lazy' test that does not enumerate all choices
        return item<=self.argmax
    def __iter__(self):
        # iterable for display in error message
        # use is in:
        # tup = value, ', '.join(map(repr, action.choices))
        # metavar bypasses this when formatting help/usage
        return iter(['integers less than %s'%self.argmax])

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--regular','-r',choices=['one','two'])
larg = parser.add_argument('--lazy','-l', choices=LazyChoice(10))
larg.type = int
print parser.parse_args()

Implementing the testing part (__contains__) is easy. The help/usage can be customized with help and metavar attributes. Customizing the error message is harder. http://bugs.python.org/issue16468 discusses alternatives when choices are not iterable. (also on long list choices: http://bugs.python.org/issue16418)

I've also shown how the type can be changed after the initial setup. That doesn't solve the problem of setting type based on subparser choice. But it isn't hard to write a custom type, one that does some sort of Db lookup. All a type function needs to do is take a string, return the correct converted value, and raise ValueError if there's a problem.