Python - Validator Action for argparse Module

Python's argparse module is quite featureful, readble, well-documented and easy to use. Below is an simple example:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('somearg', help='Some argument')

args = parser.parse_args()
parameter = args.somearg
print("You provided parameter {}!".format(parameter))

and run it.

$ python3 test.py
usage: test.py [-h] somearg
test.py: error: the following arguments are required: somearg

$ python3 test.py cat
You provided parameter cat!

The argparse module automatically handles help which is pretty handy

$ python3 test.py -h
usage: test.py [-h] somearg

positional arguments:
  somearg     Some argument

optional arguments:
  -h, --help  show this help message and exit

Actions:

You can create a custom action if you want to validate the parameter on the fly! ArgumentParser objects associate command-line arguments with actions. These actions can do just about anything with the command-line arguments associated with them. The action keyword argument specifies how the command-line arguments should be handled. Other than validator, here are list of supplied actions:

• store - This just stores the argument’s value
• store_const - This stores the value specified by the const keyword argument
• store_true and store_false - These are special cases of 'store_const' used for storing the values True and False respectively
• append - This stores a list, and appends each argument value to the list
• append_const - This stores a list, and appends the value specified by the const keyword argument to the list
• count - This counts the number of times a keyword argument occurs
• help - This prints a complete help message for all the options in the current parser and then exits
• version - This expects a version= keyword argument in the add_argument() call, and prints version information

This blog only covers customized validators.

Define a validator class:

class ValidateParameter(argparse.Action):
    """Validate parameter"""
    def __call__(self, parser, namespace, values, option_string=None):
        if not values.isupper():
            sys.exit("This parameter must be upper case")
        else:
            setattr(namespace, self.dest, values)

Then when you add the argument, you can insert the action which calls the "ValidateParameter" we just defined.
parser.add_argument('somearg', action=ValidateParameter, help='Some argument')

Run it again:

$ python3 test.py lowercase
This parameter must be uppercase
$ python3 test.py LOWERCASE
You provided parameter LOWERCASE!

The source code:

import argparse
import sys

class ValidateParameter(argparse.Action):
    """Validate parameter"""
    def __call__(self, parser, namespace, values, option_string=None):
        if not values.isupper():
            sys.exit("This parameter must be uppercase")
        else:
            setattr(namespace, self.dest, values)

parser = argparse.ArgumentParser()
parser.add_argument('somearg', action=ValidateParameter, help='Some argument')

args = parser.parse_args()
parameter = args.somearg
print("You provided parameter {}!".format(parameter))