argparse: TypeError when adding an argument using parameters store_true, default and type

[nessita]

I'm building a custom argparse/configparser combined config manager to replace a deprecated dependency used in a project I'm porting to Python 3 (I did find ConfigArgParse though I would rather to not use a third party lib for this yet).

When constructing the ArgumentParser and adding arguments to it, I was puzzled by the following exception being raised after calling add_argument:

TypeError: __init__() got an unexpected keyword argument 'type'

The minimal code to reproduce the error I got is:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--foo', default='False', action='store_true', type=bool)

Complete exception:

/usr/lib/python3.8/argparse.py in add_argument(self, *args, **kwargs)
   1378         if not callable(action_class):
   1379             raise ValueError('unknown action "%s"' % (action_class,))
-> 1380         action = action_class(**kwargs)
   1381
   1382         # raise an error if the action type is not callable

TypeError: __init__() got an unexpected keyword argument 'type'

After some investigation, I found out that the code raising this exception is the argparse._StoreTrueAction when called with the parameters passed to add_argument. I did some googling and I did find this related stackoverflow post, where the OP seems to be confused by wanting to use store_true with int/str options. My case is different in that my options are indeed bool options, and I do want the parser to store_true when the option is passed, and to treat the default 'False' as bool.

A fair question you may ask is why my default is the string 'False' instead of the False value: this is a side-effect of grabbing all the config options from config files (using ConfigParser), and having the default values being fetched as strings. While I could convert this value to bool before passing it to add_argument, I was making use of the following documented feature of argparse:

If the default value is a string, the parser parses the value as if it were a command-line argument. In particular, the parser applies any type conversion argument, if provided, before setting the attribute on the Namespace return value.

I'm aware this can be solved with a workaround in my code, and in a way the 'False' default is incorrect, so I would totally understand if this issue is closed as Invalid. Thanks!

• CPython versions tested on: 3.8
• Operating system and architecture: Linux 5.4.0-124-generic x86_64

[mahdy786]

Hi Nessit
The problem your facing has already been solved here https://stackoverflow.com/questions/33574270/typeerror-init-got-an-unexpected-keyword-argument-type-in-argparse

[nessita]

Hi @mahdy786, thank you for trying to help. I have referenced that stackoverflow post in my bug report, and as I mentioned originally, it does not quite match the issue I'm having. In the post, the OP is operating with strings and ints, so store_true (or store_false) make no sense. In my use case, I'm indeed working with booleans and I'd like to use both type and store_true for the reasons explained above.

[hpaulj]

The use, or non-use, of the 'type=bool' is documented under 'type'. bool('False') will not do what you want.

In the linked SO, the issue isn't type=int. Any use of type with 'store_true' will raise this error. The error is raised by signature of the _StoreTrueAction class. It does not accept type as a keyword parameter.

This action subclass does not take any arguments (nargs=0), so there's nothing to convert via the type. So it doesn't expect or need this parameter.

We could get around this TypeError by setting the type after the add_argument. But as this demonstrates, bool('False') is still True:

 In [78]: p = argparse.ArgumentParser()
 In [79]: a = p.add_argument('--foo',action='store_true', default="False")  
 In [80]: a.type=bool
 In [81]: p.parse_args([])
 Out[81]: Namespace(foo=True)])

You could write your own custom type function that converts the string 'False' to boolean False. That kind of function has been suggested in other bug/issues, but we've rejected making it part of argparse. But I think the real solution is to make your 'config grabber' smarter. 'store_true' cannot, and should not, accept a type parameter.

[nessita]

Thank you @hpaulj for your response. I do understand that currently store_true is the one raising the error and that _StoreTrueAction does not accept type as keyword parameter. Perhaps I could file a new issue for the error to be clearer and a bit more user friendly? (it took me a while to understand what was going on).

And indeed I have written my own custom type function that converts the string 'False' to boolean False (I called it boolean_parser in my solution below). Given your response, I guess we can close this issue as invalid?

In any case, for any future reader that has the same issue, I'm sharing my workaround here:

def boolean_parser(value):
    value = getattr(value, 'lower', lambda: None)()
    if value in ('1', 'yes', 'true', 'on'):
        result = True
    elif value in ('0', 'no', 'false', 'off'):
        result = False
    else:
        raise argparse.ArgumentTypeError(
            '%r is not a valid boolean-like value.' % value
        )
    return result


class _StoreTrueAction(argparse.Action):
    """Re-implement from upstream.

    https://github.com/python/cpython/issues/96220

    """

    def __init__(
        self,
        option_strings,
        dest,
        default=False,
        type=boolean_parser,
        required=False,
        help=None,
        metavar=None,
    ):
        super(_StoreTrueAction, self).__init__(
            option_strings=option_strings,
            dest=dest,
            nargs=0,
            const=True,
            default=default,
            type=type,
            required=required,
            help=help,
            metavar=metavar,
        )

    def __call__(self, parser, namespace, values, option_string=None):
        setattr(namespace, self.dest, self.const)

[serhiy-storchaka]

Closing as this is not a bug in argparse. The store_true action cannot be used with type because it does not take any value.