`Sentinel` is passed to `type_cast_value`

[reallistic]

Starting in version 8.3.0, a Sentinel object is passed in to the type_cast_value function of the Parameter class and its derivatives if no value is supplied.

Reproduction:

# cli_bug.py
import click


class CSVOption(click.Option):
    def type_cast_value(self, ctx, value):
        if value:
            return value.split(",")
        return value


@click.command(name="run")
@click.option("--csv", cls=CSVOption)
def main(csv):
    click.echo(f"{csv!r}")

if __name__ == "__main__":
    main()

$ uv run --with 'click==8.3.0' cli_bug.py

Installed 1 package in 1ms
Traceback (most recent call last):
  File "/home/coder/workspace/scratch/click-8_3_0-bug/cli_bug.py", line 17, in <module>
    main()
  File "/home/coder/.cache/uv/archive-v0/9Q5N3REh-YZUOJbO3iHRv/lib/python3.10/site-packages/click/core.py", line 1462, in __call__
    return self.main(*args, **kwargs)
  File "/home/coder/.cache/uv/archive-v0/9Q5N3REh-YZUOJbO3iHRv/lib/python3.10/site-packages/click/core.py", line 1382, in main
    with self.make_context(prog_name, args, **extra) as ctx:
  File "/home/coder/.cache/uv/archive-v0/9Q5N3REh-YZUOJbO3iHRv/lib/python3.10/site-packages/click/core.py", line 1206, in make_context
    self.parse_args(ctx, args)
  File "/home/coder/.cache/uv/archive-v0/9Q5N3REh-YZUOJbO3iHRv/lib/python3.10/site-packages/click/core.py", line 1217, in parse_args
    _, args = param.handle_parse_result(ctx, opts, args)
  File "/home/coder/.cache/uv/archive-v0/9Q5N3REh-YZUOJbO3iHRv/lib/python3.10/site-packages/click/core.py", line 2516, in handle_parse_result
    value = self.process_value(ctx, value)
  File "/home/coder/.cache/uv/archive-v0/9Q5N3REh-YZUOJbO3iHRv/lib/python3.10/site-packages/click/core.py", line 2401, in process_value
    value = self.type_cast_value(ctx, value)
  File "/home/coder/workspace/scratch/click-8_3_0-bug/cli_bug.py", line 7, in type_cast_value
    return value.split(",")
AttributeError: 'Sentinel' object has no attribute 'split'

uv run --with 'click<8.3.0' cli_bug.py
None

Based on comments in #3065 and #3068 it would make sense to pass through None here given that this function is public. At a minimum, a changelog denoting the new behavior would be nice.

Environment:

• Python version: 3.10
• Click version: 8.3.0

[Rowlando13]

Thanks for the clear report. Should be None.

[Rowlando13]

@sirosen Thanks for helping with #3079 Would you mind taking a look at this?

[sirosen]

Yeah, I'll see if I can work out a fix for this one.

[Rowlando13]

Great!