How can I require my python script's argument to be a float in a range using argparse?

Question

I'd like to use argparse on Python 2.7 to require that one of my script's parameters be between the range of 0.0 and 1.0. Does argparse.add_argument() support this?

Answer 1

The type parameter to add_argument just needs to be a callable object that takes a string and returns a converted value. You can write a wrapper around float that checks its value and raises an error if it is out of range.

def restricted_float(x):
    try:
        x = float(x)
    except ValueError:
        raise argparse.ArgumentTypeError("%r not a floating-point literal" % (x,))

    if x < 0.0 or x > 1.0:
        raise argparse.ArgumentTypeError("%r not in range [0.0, 1.0]"%(x,))
    return x

p = argparse.ArgumentParser()
p.add_argument("--arg", type=restricted_float)

Answer 2

Here is a method that uses the choices parameter to add_argument, with a custom class that is considered "equal" to any float within the specified range:

import argparse

class Range(object):
    def __init__(self, start, end):
        self.start = start
        self.end = end
    def __eq__(self, other):
        return self.start <= other <= self.end

parser = argparse.ArgumentParser()
parser.add_argument('--foo', type=float, choices=[Range(0.0, 1.0)])

Answer 3

Adding str makes that the boundaries are visuable in the help.

import argparse

class Range(object):
    def __init__(self, start, end):
        self.start = start
        self.end = end

    def __eq__(self, other):
        return self.start <= other <= self.end

    def __contains__(self, item):
        return self.__eq__(item)

    def __iter__(self):
        yield self

    def __str__(self):
        return '[{0},{1}]'.format(self.start, self.end)

parser = argparse.ArgumentParser()
parser.add_argument('--foo', type=float, choices=Range(0.0, 1.0))
parser.add_argument('--bar', type=float, choices=[Range(0.0, 1.0), Range(2.0,3.0)])

Answer 4

The argparse.add_argument call expects an iterable as 'choices' parameter. So what about adding the iterable property to the Range class above. So both scenarios could be used:

import argparse

class Range(object):
    def __init__(self, start, end):
        self.start = start
        self.end = end

    def __eq__(self, other):
        return self.start <= other <= self.end

    def __contains__(self, item):
        return self.__eq__(item)

    def __iter__(self):
        yield self

parser = argparse.ArgumentParser()
parser.add_argument('--foo', type=float, choices=Range(0.0, 1.0))
parser.add_argument('--bar', type=float, choices=[Range(0.0, 1.0), Range(2.0,3.0)])