How to Parse Boolean Command-Line Arguments with argparse: Why \"--foo False\" Doesn\'t Always Mean False?

Parsing Boolean Command-Line Arguments with argparse

When using argparse to parse boolean arguments, users may encounter challenges if they input values as "--foo True" or "--foo False." Despite the intuitive assumption that "--foo False" should evaluate to False, argparse interprets it as True. This behavior also occurs when inputting an empty string for the boolean argument.

To address this issue, argparse provides alternative parsing options for boolean flags:

1. Action=argparse.BooleanOptionalAction (Python 3.9 )

By specifying action=argparse.BooleanOptionalAction, argparse will automatically interpret the presence of the flag without an argument as True, and its absence as False. For example:

<code class="python">parser.add_argument('--feature', action=argparse.BooleanOptionalAction)</code>

2. Store_True and Store_False

For Python versions prior to 3.9, the following approach can be used:

<code class="python">parser.add_argument('--feature', action='store_true')
parser.add_argument('--no-feature', dest='feature', action='store_false')
parser.set_defaults(feature=True)</code>

This allows users to enable or disable a feature by passing "--feature" or "--no-feature," respectively.

3. Custom Type Function

If desired, a custom type function can be used to handle boolean parsing. Here's an example:

<code class="python">def t_or_f(arg):
    ua = str(arg).upper()
    if 'TRUE'.startswith(ua):
        return True
    elif 'FALSE'.startswith(ua):
        return False
    else:
        raise ValueError("Invalid input")</code>

This function can then be used as the type argument when adding the boolean argument to the parser.

By utilizing these methods, users can effectively parse boolean command-line arguments and ensure that values like "False" are interpreted correctly.

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