build: add flag_alias definition for Starlarkification of python flags

[aranguyen]

With this change, when user build with a native python flag, flag_alias will point to the starlark version of the flag. Please see design doc here for more details.

This feature should work as a no-op in the latest bazel 8.x and 7.x releases.

[gemini-code-assist]

Summary of Changes

Hello @aranguyen, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request integrates flag_alias definitions into the MODULE.bazel file, a critical step in migrating Python build flags to a Starlark-based configuration system. By mapping existing native flags to their new Starlark implementations, the change ensures a smooth transition and consistent behavior for users while advancing the overall Starlarkification initiative.

Highlights

• Starlarkification of Python Flags: This pull request adds flag_alias definitions to MODULE.bazel to facilitate the Starlarkification of Python-related build flags.
• Flag Aliases Added: Specific aliases have been introduced for build_python_zip, incompatible_default_to_explicit_init_py, python_path, and experimental_python_import_all_repositories.
• Mapping to Starlark Versions: These aliases ensure that when native Python flags are used, they correctly point to their Starlark-defined counterparts located under //python/config_settings.

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[gemini-code-assist]

Code Review

The pull request successfully introduces flag_alias definitions for several native Python flags, mapping them to their Starlark counterparts. This change is crucial for the ongoing Starlarkification effort, ensuring that users can continue to build with these flags while transitioning to Starlark-based implementations. The added aliases (build_python_zip, incompatible_default_to_explicit_init_py, python_path, and experimental_python_import_all_repositories) correctly point to their respective build settings in //python/config_settings. The changes are well-aligned with the stated objective of the pull request and the design document.

[rickeylev]

Is flag_alias bazel 9+? All the CI jobs are failing because it's not available.

At the least, we need to support 8.x.

We can drop 7.x pretty soon -- I'm thinking the Jan release?

[gregestren]

This should work as a no-op in the latest bazel 8.x and 7.x releases.

i.e. they continue to read native flags as usual, so they parse these new entries but just skip over them.

[gregestren]

See bazelbuild/bazel#27337 and bazelbuild/bazel#27334.

[rickeylev]

flag_alias() should work with latest 7.x and 8.x releases

Oh ok, that explains. Thanks. I updated the PR and it's using 8.5, but it still fails, error:

ERROR: /workdir/MODULE.bazel:460:11: Traceback (most recent call last):
	File "/workdir/MODULE.bazel", line 460, column 11, in <toplevel>
		flag_alias("build_python_zip", "//python/config_settings:build_python_zip")
Error in flag_alias: flag_alias() got unexpected positional argument

[gregestren]

@aranguyen I think you want to give the parameters names to fix that.

[aignas]

Please also add a changelog note stating the minimum supported bazel versions coming with this change.

[shayanhoshyari]

will this work for users of rules_python too?
otherwise I think this issue still stands : #3252 (comment)

[gregestren]

That should work today, even before this PR is submitted.

Bazel 9 hard-codes those aliases right now. Once this PR works we'll remove the hard-coded logic from bazel@head.

The intention is users should have a seamless experience regardless of what's going on behind the scenes.

[shayanhoshyari]

Thanks! I read https://docs.google.com/document/d/1yOvi4hVV7Ja32ocwVb4lsEUnijftk8nilXPncYm-BH8/edit?tab=t.0#heading=h.mfw8nr4w6amb briefly, and as you say this trickles down to users too, and not just rules_python. Makes sense.

Now another question, by adding this PR, should this code just be removed now?

rules_python/python/private/flags.bzl

Lines 39 to 73 in 144984c

	_POSSIBLY_NATIVE_FLAGS = {
	"build_python_zip": (lambda ctx: ctx.fragments.py.build_python_zip, "native"),
	"default_to_explicit_init_py": (lambda ctx: ctx.fragments.py.default_to_explicit_init_py, "native"),
	"python_import_all_repositories": (lambda ctx: ctx.fragments.bazel_py.python_import_all_repositories, "native"),
	"python_path": (lambda ctx: ctx.fragments.bazel_py.python_path, "native"),
	}
	def read_possibly_native_flag(ctx, flag_name):
	"""
	Canonical API for reading a Python build flag.
	Flags might be defined in Starlark or native-Bazel. This function reasd flags
	from tbe correct source based on supporting Bazel version and --incompatible*
	flags that disable native references.
	Args:
	ctx: Rule's configuration context.
	flag_name: Name of the flag to read, without preceding "--".
	Returns:
	The flag's value.
	"""
	# Bazel 9.0+ can disable these fragments with --incompatible_remove_ctx_py_fragment and
	# --incompatible_remove_ctx_bazel_py_fragment. Disabling them means bazel expects
	# Python to read Starlark flags.
	use_native_def = hasattr(ctx.fragments, "py") and hasattr(ctx.fragments, "bazel_py")
	# Developer override to force the Starlark definition for testing.
	if _POSSIBLY_NATIVE_FLAGS[flag_name][1] == "starlark":
	use_native_def = False
	if use_native_def:
	return _POSSIBLY_NATIVE_FLAGS[flag_name][0](ctx)
	else:
	# Starlark definition of "--foo" is assumed to be a label dependency named "_foo".

(from #3290) and just the rules_python config be read instead? Because the flag_alias is already doing (and even a better job) than what this code is trying to do.

[gregestren]

That should still be necessary so rules_python remains compatible with bazel 7 and 8, which can't resolve the flag aliases. Whenever rules_python drops support for pre-9 bazel we can remove this code.