TL;DR

If you want to set parameter’s default value to True, use

parser.add_argument('--param', action='store_false')

Otherwise, use

parser.add_argument('--param', action='store_true')

when we use argparse to manager program parameters and add a parameter, we often use the action options to set the parameter’s value to a boolean value. But I have previously misunderstood this options and made a mistake. In this post, I will explain how does this option work and how to use it correctly.

Typically we use action='store_true' or action='store_false' to set the parameter to boolean value. Previously, I thought that action='store_true' will set the parameter value to True and action='store_false' will set the parameter value to False. In fact, it is not true. Suppose we have a script toy_parser.py:

import argparase

parser = argparse.ArgumentParser(description="A toy argument parser")
parser.add_argument('--param1', action='store_true')
parser.add_argument('--param2', action='store_false')

args = parser.parse_args()

print(args)

If we run this script on the command line with no argument

python toy_parser.py

the output will be:

Namespace(param1=False, param2=True)

The result is exactly contratry to what we thought.

What about run the script with parameters?

$ python toy_parser.py –param1 –param2
Namespace(param1=True, param2=False)

$ python toy_parser.py –param1
Namespace(param1=True, param2=True)

$ python toy_parser.py –param2
Namespace(param1=False, param2=False)

In summary, if a parameter has action store_true but is missing in the command line, the default value will be False, otherwise, the value will be True. Similarly, if a parameter has action store_false but is missing in the command line, the default value will be True, otherwise, the value will be False.

The following two statements are problematic:

parser.add_argument('--param1', action='store_true', default=True)
parser.add_argument('--param2', action='store_false', default=False)

Wheter you use these paramters on the command line, they will always be True or False, which is not what we want.