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

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)