Why Does \"--foo False\" Evaluate to True When Parsing Boolean Arguments with argparse?

Parsing Boolean Values with argparse

Question:

When parsing boolean command-line arguments with argparse, why do values like "--foo False" evaluate to True instead of False?

Answer:

Canonical Method:

The recommended approach is to use the following format:

command --feature

For negating the feature, use:

command --no-feature

argparse provides built-in support for this:

• Python 3.9 : parser.add_argument('--feature', action=argparse.BooleanOptionalAction)

• Python < 3.9:

  parser.add_argument('--feature', action='store_true')
  parser.add_argument('--no-feature', dest='feature', action='store_false')
  parser.set_defaults(feature=True)

Alternative Method for Custom Parsing:

If the "--foo True/False" format is preferred, one option is to use ast.literal_eval or a custom function as the type:

import ast

def t_or_f(arg):
    ua = str(arg).upper()
    if 'TRUE'.startswith(ua):
        return True
    elif 'FALSE'.startswith(ua):
        return False
    else:
        pass  # Handle error condition appropriately

<code class="python">parser.add_argument("--my_bool", type=ast.literal_eval)
parser.add_argument("--my_bool", type=t_or_f)</code>

This custom function interprets uppercase True/False as boolean values, allowing for flexible parsing of these values.

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