Inconsistent completion when using a custom validator between fish and bash

[waivek]

Code Sample for Reproduction:

#!/usr/bin/env /home/vivek/pyscripts/.direnv/python-3.10.12/bin/python
# PYTHON_ARGCOMPLETE_OK

import argparse
import argcomplete

def complete_foo(prefix, parsed_args, **kwargs):
    return ['foo', 'foobar', 'foo1', 'foo2', 'red']

def default_validator(completion_candidate, current_input):
    return completion_candidate.startswith(current_input)

def custom_validator(completion_candidate, current_input):
    if current_input == "x":
        return True
    return default_validator(completion_candidate, current_input)

parser = argparse.ArgumentParser()
parser.add_argument('foo').completer = complete_foo # pyright: ignore[reportAttributeAccessIssue]

argcomplete.autocomplete(parser, validator=custom_validator)
args = parser.parse_args()

To register completions on bash, I do

chmod +x
eval "$(register-python-argcomplete mycommand.py)"

And for fish I run

register-python-argcomplete --shell fish /path/to/mycommand.py | source

On bash, when I type ./mycommand.py x<TAB> I get:

This is the desired result, that I would also like to get in my fish completion, however on fish I get:

Basically, no matches.

Upon more experimentation it seems that the fish completion script seems to be adding it's own startswith type check, thus overriding the custom validator.

Is there a way to modify the generated fish completion script to remove this behavior?

[evanunderscore]

I'm not sure exactly what you mean when you say "the fish completion script seems to be adding it's own startswith type check, thus overriding the custom validator." This is the code we're generating:

argcomplete/argcomplete/shell_integration.py

Lines 95 to 112 in 6c9e540

	fishcode = r"""
	function __fish_%(function_name)s_complete
	set -x _ARGCOMPLETE 1
	set -x _ARGCOMPLETE_DFS \t
	set -x _ARGCOMPLETE_IFS \n
	set -x _ARGCOMPLETE_SUPPRESS_SPACE 1
	set -x _ARGCOMPLETE_SHELL fish
	set -x COMP_LINE (commandline -p)
	set -x COMP_POINT (string length (commandline -cp))
	set -x COMP_TYPE
	if set -q _ARC_DEBUG
	%(argcomplete_script)s 8>&1 9>&2 1>&9 2>&1
	else
	%(argcomplete_script)s 8>&1 9>&2 1>/dev/null 2>&1
	end
	end
	complete %(completion_arg)s %(executable)s -f -a '(__fish_%(function_name)s_complete)'
	"""

My guess would be fish's own machinery is filtering returned completions by prefix. I'm not familiar enough with fish to know if there's an option to change this. It's possible the shell simply doesn't allow it.