Python argparse: 'usage' text showing formatter object when using custom formatter

Question

I got a script with a list of arguments defined with argparse. Because I want to have bullet points and line breaks in the help text, I added a custom formatter. That works like a charm but the problem now is that the default 'usage' text in the help, now shows the formatter object:

usage: <__main__.MyFormatter object at 0x7f30cc163c70> [-h] {list,add,remove} ...

If I remove the custom formatter, then the 'usage' is as expected:

usage: test.py [-h] {list,add,remove} ...

And I can't understand why and how to fix it.

Here's a stripped down version of my script (test.py):

import argparse
class MyFormatter(argparse.ArgumentDefaultsHelpFormatter):
    def __init__(self,
            prog,
            indent_increment=2,
            max_help_position=30,
            width=None):
        super().__init__(self,
                        indent_increment=indent_increment,
                        max_help_position=max_help_position,
                        width=width)
    def _split_lines(self, text, width):
        """
        If the text starts with 'S|', remove the prefix, split the text into lines at the new line '\n'
        and maintain spaces. Otherwise, uses the default splitting behavior of ArgumentDefaultsHelpFormatter.
        """
        if text.startswith('S|'):
            text = text[2:].strip().splitlines()
            import textwrap
            out = []
            for line in text:
                out.extend(textwrap.wrap(line, width))
            return out
        return argparse.ArgumentDefaultsHelpFormatter._split_lines(self, text, width)

parser = argparse.ArgumentParser(description="This script will do a bunch of stuff",
                                formatter_class=MyFormatter)
parser.set_defaults(func=lambda args: parser.print_help())
subparsers = parser.add_subparsers()

parser_list = subparsers.add_parser("list", help="Show a list of things", formatter_class=MyFormatter)

parser_add = subparsers.add_parser("add", help="Add item to the list", formatter_class=MyFormatter)
parser_add.add_argument("item", type=str, help="Item to add to the list")

parser_remove = subparsers.add_parser("remove", help="Remove item from the list", formatter_class=MyFormatter)
parser_remove.add_argument("item", type=str, help="Item to remove from the list")

args = parser.parse_args()

I know I can add my own usage message with the 'usage=' argument, but I'm more trying to understand why using the custom formatter breaks the usage text?

PS. using Python 3.12

You passed self instead of prog to the superclass constructor. — user2357112
– user2357112, Commented Mar 13 at 23:26
Holy ****! You are right! I completely missed that! Thank you, it was driving me crazy! — stefanos
– stefanos, Commented Mar 14 at 6:17

Hai Vu · Accepted Answer · 2025-09-30 00:54:32Z

0

As mentioned, you passed self in place of prog. There are two fixes:

Replace self with prog

    def __init__(self, prog, indent_increment=2, max_help_position=30, width=None):
        super().__init__(
            prog,
            indent_increment=indent_increment,
            max_help_position=max_help_position,
            width=width,
        )

Since the __init__ function does not add customization, go ahead and delete it.

answered Sep 30 at 0:54

Hai Vu

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