build: add flag_alias definition for Starlarkification of python flags by aranguyen · Pull Request #3450 · bazel-contrib/rules_python

aranguyen · 2025-12-10T15:39:19Z

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 · 2025-12-10T15:39:35Z

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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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 · 2025-12-10T16:51:59Z

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 · 2025-12-12T23:18:10Z

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 · 2025-12-12T23:19:48Z

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

rickeylev · 2025-12-14T23:08:51Z

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 · 2025-12-15T14:48:23Z

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

aignas · 2025-12-16T14:32:03Z

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

shayanhoshyari · 2026-01-08T22:31:49Z

MODULE.bazel

+flag_alias(name = "build_python_zip", starlark_flag = "//python/config_settings:build_python_zip")
+flag_alias(name = "incompatible_default_to_explicit_init_py", starlark_flag = "//python/config_settings:incompatible_default_to_explicit_init_py")
+flag_alias(name = "python_path", starlark_flag = "//python/config_settings:python_path")
+flag_alias(name = "experimental_python_import_all_repositories", starlark_flag = "//python/config_settings:experimental_python_import_all_repositories")


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

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.

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.

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.

shayanhoshyari · 2026-01-10T18:02:17Z

tests/base_rules/py_executable_base_tests.bzl

            # Pass value to both native and starlark versions of the flag until
            # the native one is removed.
-            "//command_line_option:build_python_zip": "true",
+            "//python/config_settings:build_python_zip": "true",


@gregestren sorry for ping again. Are you sure this is a correct change on bazel 7 and 8?

My understanding is that

you mentioned flag alias is no op on these versions.

and the code here

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".

will always pick the py_fragments values and ignore the rules_python configs for bazel 7 and 8.

So the result is this change to overrides on these bazel versions will make the override stop overriding anything, breaking the test.

I see in CI failures like below that probably is related

(18:22:56) ERROR: /workdir/tests/config_settings/transition/BUILD.bazel:6:25: On dependency edge //tests/config_settings/transition:test_py_binary_windows_build_python_zip_false (722907e) -|target|-> //tests/config_settings/transition:test_py_binary_windows_build_python_zip_false_subject: com.google.devtools.build.lib.packages.BuildFileNotFoundException: no such package '@@rules_testing//python/config_settings': BUILD file not found in directory 'python/config_settings' of external repository @@rules_testing. Add a BUILD file to a directory to mark it as a package. (18:22:56) ERROR: Analysis of target '//tests/config_settings/transition:test_py_binary_windows_build_python_zip_false' failed; build aborted

Hi @shayanhoshyari , taking a closer look at the error no such package '@@rules_testing//python/config_settings': BUILD file not found in directory 'python/config_settings' of external repository @@rules_testing

I'm wondering if this is just changing this from "//python/config_settings:build_python_zip": "true", to "@@rules_python//python/config_settings:build_python_zip": "true",?

The code seems to be looking for this flag definition in package @@rules_testing

and probably update @@rules_testing to depends on the newer version of @rules_python that has these flag def? https://github.com/bazelbuild/rules_testing/blob/master/MODULE.bazel#L19

add flag_alias definition for Starlarkification of python flags

7b52502

aranguyen requested review from aignas and rickeylev as code owners December 10, 2025 15:39

gemini-code-assist bot reviewed Dec 10, 2025

View reviewed changes

Merge branch 'main' into main

e9688b7

aranguyen and others added 5 commits December 18, 2025 10:20

Merge branch 'bazel-contrib:main' into main

15687f1

fix error

5c6539e

use starlark flags instead

504ccf3

fix starlark label

effbe4c

remove extra character

e7bc464

shayanhoshyari reviewed Jan 8, 2026

View reviewed changes

shayanhoshyari reviewed Jan 10, 2026

View reviewed changes

aranguyen and others added 11 commits January 12, 2026 14:47

Merge branch 'bazel-contrib:main' into main

ee233fd

use common label

84439af

remove incorrect label

3c53c52

update bazel version to 7.7.0

0dbccff

fix

0c7134c

update bazel version to 8.5.1 where flag_alias is backported as no-op

7106a6b

add back the native flag reference as that is needed

e630cb8

add the correct flags

25895ab

add correct? label for transition

5fd0240

use apparent repo label

c67c288

dummy commit

f9ac7d2

	_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".

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