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How to use Python 'argparse': Default Values, Optional Arguments, and nargs='?'

Learn how to create optional command-line arguments in Python argparse using nargs='?', const, and default for flexible default values when arguments are omitted or provided without values.

Python's argparse module provides a powerful way to create command-line interfaces.

This guide explains how to use nargs='?', const, and default in argparse to create optional arguments that have default values if the argument is either not provided at all, or is provided without a value.

Understanding nargs='?', const, and default

When working with argparse, these three arguments are often used together to define optional arguments with defaults:

  • nargs='?': This makes the command-line argument optional.
    • If the argument is provided with a value (e.g., --fruit banana), that value is used.
    • If the argument is provided without a value (e.g., --fruit), the value from const is used.
    • If the argument is not provided at all (e.g., no --fruit), the value from default is used.
  • const: The value to use if the argument is present on the command line but without a value. This is only relevant when nargs='?'.
  • default: The value to use if the argument is not present on the command line at all.

Example: Optional String Argument with Default

import argparse

parser = argparse.ArgumentParser(
    description='A sample Argument Parser.'
)

parser.add_argument('-f',
                    '--fruit',
                    nargs='?',          # Argument is optional
                    const='apple',      # Value if --fruit is present but has no value
                    default='apple',    # Value if --fruit is NOT present
                    type=str
                    )

args = parser.parse_args()
print(args.fruit)

Let's break down the possible command-line invocations and their results:

Command Line Output Explanation
python main.py apple --fruit is not present, so default='apple' is used.
python main.py --fruit apple --fruit is present, but no value is given, so const='apple' is used.
python main.py --fruit banana banana --fruit is present and a value is given, so the provided value (banana) is used. const and default are ignored.
  • If the default value is not specified, and the user does not pass any value when calling the script, then the default value will be None.

Example: Optional Integer Argument with Default

import argparse

parser = argparse.ArgumentParser(
    description='A sample Argument Parser.'
)

parser.add_argument('-n',
                    '--number',
                    nargs='?',
                    const=0,     # Value if --number is present but has no value
                    default=0,   # Value if --number is NOT present
                    type=int
                    )

args = parser.parse_args()
print(args.number)

The command line behavior will be analogous to the string example:

Command Line Output Explanation
python main.py 0 --number is not present; default=0 is used.
python main.py --number 0 --number is present, no value; const=0 is used.
python main.py --number 100 100 --number is present with a value; that value is used.

Example: Different const and default values.

It is also possible for the const and default arguments to have different values:

import argparse

parser = argparse.ArgumentParser(
    description='A sample Argument Parser.'
)
parser.add_argument('-f',
                   '--fruit',
                    nargs='?',
                    const='apple',
                    default='pear',
                    type=str
                    )

args = parser.parse_args()
print(args.fruit)

In the example above:

  • If the user doesn't pass the argument at all, default value pear is used.
  • If the user provides only the flag -f, then the const value apple is used.
  • If the user provides a value to the flag -f value, then that value is used.
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