…3.15

See python/cpython#149086 and PEP 661.

…3.15

See python/cpython#149086 and PEP 661.

…3.15

See python/cpython#149086 and PEP 661.

…thon 3.15 (#4211)

See python/cpython#149086 and [PEP
661](https://docs.python.org/3.15/whatsnew/3.15.html#whatsnew315-sentinel)
for context.

This fixes a traceback from `snakemake --help` in Python 3.15.0b1:

```
$ git clone https://github.com/snakemake/snakemake.git
$ cd snakemake
$ uv venv --python 3.15
$ . .venv/bin/activate
(snakemake) $ uv pip install -e .
(snakemake) $ snakemake --help
Traceback (most recent call last):

  File "/usr/lib64/python3.15/argparse.py", line 2878, in print_help
    help_text = self.format_help(formatter=formatter)

TypeError: ArgumentParser.format_help() got an unexpected keyword argument 'formatter'


During handling of the above exception, another exception occurred:


Traceback (most recent call last):

  File "/home/ben/src/forks/snakemake/src/snakemake/cli.py", line 2391, in main
    parser, args = parse_args(argv)
                   ~~~~~~~~~~^^^^^^

  File "/home/ben/src/forks/snakemake/src/snakemake/cli.py", line 1904, in parse_args
    args = parser.parse_args(argv)

  File "/home/ben/src/forks/snakemake/.venv/lib64/python3.15/site-packages/configargparse.py", line 1048, in parse_args
    args, argv = self.parse_known_args(
                 ~~~~~~~~~~~~~~~~~~~~~^
        args=args,
        ^^^^^^^^^^
    ...<3 lines>...
        ignore_help_args=False,
        ^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^

  File "/home/ben/src/forks/snakemake/.venv/lib64/python3.15/site-packages/configargparse.py", line 1266, in parse_known_args
    namespace, unknown_args = argparse.ArgumentParser.parse_known_args(
                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        self, args=args, namespace=namespace
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^

  File "/usr/lib64/python3.15/argparse.py", line 2126, in parse_known_args
    return self._parse_known_args2(args, namespace, intermixed=False)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/usr/lib64/python3.15/argparse.py", line 2155, in _parse_known_args2
    namespace, args = self._parse_known_args(args, namespace, intermixed)
                      ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/usr/lib64/python3.15/argparse.py", line 2406, in _parse_known_args
    start_index = consume_optional(start_index)

  File "/usr/lib64/python3.15/argparse.py", line 2330, in consume_optional
    take_action(action, args, option_string)
    ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/usr/lib64/python3.15/argparse.py", line 2231, in take_action
    action(self, namespace, argument_values, option_string)
    ~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/usr/lib64/python3.15/argparse.py", line 1279, in __call__
    parser.print_help()
    ~~~~~~~~~~~~~~~~~^^

  File "/usr/lib64/python3.15/argparse.py", line 2882, in print_help
    help_text = self.format_help()

  File "/home/ben/src/forks/snakemake/.venv/lib64/python3.15/site-packages/configargparse.py", line 1696, in format_help
    return argparse.ArgumentParser.format_help(self) + (
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^

  File "/usr/lib64/python3.15/argparse.py", line 2839, in format_help
    return formatter.format_help()
           ~~~~~~~~~~~~~~~~~~~~~^^

  File "/usr/lib64/python3.15/argparse.py", line 338, in format_help
    help = self._root_section.format_help()

  File "/usr/lib64/python3.15/argparse.py", line 263, in format_help
    item_help = join([func(*args) for func, args in self.items])
                      ~~~~^^^^^^^

  File "/usr/lib64/python3.15/argparse.py", line 263, in format_help
    item_help = join([func(*args) for func, args in self.items])
                      ~~~~^^^^^^^

  File "/usr/lib64/python3.15/argparse.py", line 612, in _format_action
    help_text = self._expand_help(action)

  File "/usr/lib64/python3.15/argparse.py", line 712, in _expand_help
    help_string = self._get_help_string(action)

  File "/home/ben/src/forks/snakemake/src/snakemake/common/argparse.py", line 76, in _get_help_string
    and not isinstance(action.default, dataclasses._MISSING_TYPE)
                                       ^^^^^^^^^^^^^^^^^^^^^^^^^

AttributeError: module 'dataclasses' has no attribute '_MISSING_TYPE'

```

The `TypeError` is supposed to happen, but it is supposed to be caught:
https://github.com/python/cpython/blob/f31a89bb901067dd105b00cfa90523cf7ffdbbdd/Lib/argparse.py#L2877-L2882;
that’s all in the Python standard library. But then the `AttributeError`
on `_MISSING_TYPE` happens, and that’s the bug/regression in Snakemake.

After this PR, `snakemake --help` works as expected. Trying again in a
Python 3.14 virtualenv, I confirm that `snakemake --help` *still* works
as expected.

There are probably 100 different ways that this PR could have been
written differently, and half of them are actually reasonable. I’m happy
to consider style and implementation changes if requested.

One thing I considered is that it might be just as good to always test
against `dataclasses.MISSING`, which [already
existed](https://docs.python.org/3.14/library/dataclasses.html#dataclasses.MISSING)
with sentinel semantics even before the introduction of the PEP 661
`sentinel` type. I think that this probably works as expected, and it’s
a lot simpler:

```diff
diff --git a/src/snakemake/common/argparse.py b/src/snakemake/common/argparse.py
index 1801419..db8d8b07 100644
--- a/src/snakemake/common/argparse.py
+++ b/src/snakemake/common/argparse.py
@@ -73,7 +73,7 @@ class ArgumentDefaultsHelpFormatter(argparse.HelpFormatter):
                 or action.nargs in [argparse.OPTIONAL, argparse.ZERO_OR_MORE]
             )
             and action.default not in (None, "", set(), argparse.SUPPRESS)
-            and not isinstance(action.default, dataclasses._MISSING_TYPE)
+            and not action.default is dataclasses.MISSING
         ):
             if isinstance(action.default, collections.abc.Iterable) and not isinstance(
                 action.default, str
```

It’s worth considering that both of these approaches rely on
implementation details of `dataclasses.MISSING`, and either of them may
break at any point in the future. See the warning in
https://docs.python.org/3.14/library/dataclasses.html#dataclasses.field
that “No code should directly use the `MISSING` value.” Maybe taking a
step back and reflecting on the bigger picture could reveal a better
approach that sidesteps the issue completely. I’m not prepared to spend
any more time on it right now, so please take this PR as a combined bug
report and initial suggestion.

<!--Add a description of your PR here-->

### QC
<!-- Make sure that you can tick the boxes below. -->

* [ ] The PR contains a test case for the changes or the changes are
already covered by an existing test case. **To be honest, I don’t think
there is anything in the test suite that checks that `snakemake --help`
doesn’t crash, but I’m not prepared to contribute a test case. I welcome
the addition of a relevant test case to this PR, but my own contribution
will be limited to fixing the regression and confirming the fix
manually.**
* [x] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake). **not
necessary**
* [x] I, as a human being, have checked each line of code in this pull
request

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Ensure command-line help consistently recognizes and displays
missing/default argument values across Python versions, preventing
incorrect omission or duplication of default values in help text.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->