Default value of False cast to 'False' and is thus True

[jaraco]

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Example Code

import typer


def main(do_bad_thing=False):
    if do_bad_thing:
        raise NotImplementedError


__name__ == '__main__' and typer.run(main)

Description

When running a script without type annotations, but a default boolean value, I sort-of expected the type to be inferred from the default. That's how autocommand works. If the default value is a boolean and no type annotation is given, the type is inferred to be bool. But in Typer, the type presumably defaults to str, so when running the above script with no parameters, do_bad_thing is set to 'False', which is True, and a NotImplementedError is raised.

Obviously, this unexpected behavior can be fixed by always supplying type annotations, but since the issue happens silently, it's a bit of a foot gun.

Better would be to either use the default value to infer that the type is a bool or warn or raise an Exception if the default value is a bool but no type annotation was given. It's unlikely if the user is supplying a default value of True or False that they want that cast to a string.

I note also that this behavior differs from how arg = None is handled, which apparently does not get cast to a string; it seems to be handled as str | None based on the default of None.

This issue came up in jaraco.develop, where after migrating to Typer in jaraco/jaraco.develop#21, I ran the update-projects script, but it failed when dry_run was set to 'False' (True).

Operating System

macOS

Operating System Details

n/a

Typer Version

0.12.4

Python Version

3.13.0rc1

Additional Context

No response