diff --git a/Doc/howto/argparse.rst b/Doc/howto/argparse.rst index 30d9ac70037..1efbee64d60 100644 --- a/Doc/howto/argparse.rst +++ b/Doc/howto/argparse.rst @@ -841,6 +841,53 @@ translated messages. To translate your own strings in the :mod:`argparse` output, use :mod:`gettext`. +Custom type converters +====================== + +The :mod:`argparse` module allows you to specify custom type converters for +your command-line arguments. This allows you to modify user input before it's +stored in the :class:`argparse.Namespace`. This can be useful when you need to +pre-process the input before it is used in your program. + +When using a custom type converter, you can use any callable that takes a +single string argument (the argument value) and returns the converted value. +However, if you need to handle more complex scenarios, you can use a custom +action class with the **action** parameter instead. + +For example, let's say you want to handle arguments with different prefixes and +process them accordingly:: + + import argparse + + parser = argparse.ArgumentParser(prefix_chars='-+') + + parser.add_argument('-a', metavar='', action='append', + type=lambda x: ('-', x)) + parser.add_argument('+a', metavar='', action='append', + type=lambda x: ('+', x)) + + args = parser.parse_args() + print(args) + +Output: + +.. code-block:: shell-session + + $ python prog.py -a value1 +a value2 + Namespace(a=[('-', 'value1'), ('+', 'value2')]) + +In this example, we: + +* Created a parser with custom prefix characters using the ``prefix_chars`` + parameter. + +* Defined two arguments, ``-a`` and ``+a``, which used the ``type`` parameter to + create custom type converters to store the value in a tuple with the prefix. + +Without the custom type converters, the arguments would have treated the ``-a`` +and ``+a`` as the same argument, which would have been undesirable. By using custom +type converters, we were able to differentiate between the two arguments. + Conclusion ==========