argparse - Define custom actions or types with additional arguments

Question

I'm developing a toolbox containing several python scripts. For several of them some arguments may be numeric values. Depending of the script some may require a value v to be between -1 and 1, or 0 and 1 or 1 and 10 or ... An example could be a page width from an output diagram which should be always positive.

I can check all the time if v is in the required range. I could also for each of these range define an Action or a type using argparse. An example is given using a new type:

def positive_num(a_value):
    """Check a numeric positive."""
    if not a_value > 0:
        raise argparse.ArgumentTypeError("Should be positive.")
    return a_value

And add it later to the parser:

parser_grp.add_argument('-pw', '--page-width',
                        help='Output pdf file width (e.g. 7 inches).',
                        type=positive_num,
                        default=None,
                        required=False)

Now, if the value is a correlation coefficient (or anything in a range) would it be possible using action or types to write something more general using:

def ranged_num(a_value, lowest=-1, highest=1):
    """Check a numeric is in expected range."""
    if not (a_value >= lowest and a_value <= highest):
        raise argparse.ArgumentTypeError("Not in range.")
    return a_value

That could later be added like:

parser_grp.add_argument('-c', '--correlation',
                        help='A value for the correlation coefficient',
                        type=ranged_num(-1,1),
                        default=None,
                        required=False)

I have tried in several ways but whithout success.

Thank you

You need a higher-order function to do this, a function that returns a function. type needs to be a function that accepts a single (note: string) argument, so that's what ranged_num(-1, 1) needs to return. — jonrsharpe
– jonrsharpe, Commented Nov 23, 2018 at 11:28
The question is how will I pass the range (-1,1) to this higher-order function since the normal way is to pass a function definition and not a function call to the type argument of add_argument() method. I recall that I don't want to write a particular function for each range. — dputhier
– dputhier, Commented Nov 23, 2018 at 11:42
That's the point of a higher-order function; the function that returns the type function can also take parameters. You would write type=ranged_num(-1, 1), and that function would return an appropriately-configures function that accepts the string argument. — jonrsharpe
– jonrsharpe, Commented Nov 23, 2018 at 11:44
functools.partial can be used to bind the low and high of ranged_num. — hpaulj
– hpaulj, Commented Nov 23, 2018 at 16:12

jonrsharpe · Accepted Answer · 2018-11-23 11:49:50Z

7

Per the documentation:

type= can take any callable that takes a single string argument and returns the converted value

Therefore, to use it like type=ranged_num(-1,1), your ranged_num function must return a function itself. A function that returns a function (or accepts a function as an argument, or both) is often referred to as a "higher-order function".

Here's a minimal example:

def ranged_num(lowest=-1, highest=1):
    """Check a numeric is in expected range."""
    def type_func(a_value):
        a_value = int(a_value)  # or "float"; you could also have error handling here
        if not (a_value >= lowest and a_value <= highest):  # I'd rewrite this to an "or"
            raise argparse.ArgumentTypeError("Not in range.")
        return a_value
    return type_func

Now ranged_num creates and returns a function, type_func, that is responsible for handling the string coming from the command line.

answered Nov 23, 2018 at 11:49

jonrsharpe

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3 Comments

dputhier Over a year ago

I was on the way.Did not think about the fact that lowest and highest where visible inside type_func... Thanks a lot for this short lesson. Best

jonrsharpe Over a year ago

@user451460 no worries; if you want more information on that, it's referred to as a "closure"

dputhier Over a year ago

This is also nicer with a or... Thanks.

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