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build: add flag_alias definition for Starlarkification of python flags #3450

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build: add flag_alias definition for Starlarkification of python flags #3450

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7b52502
add flag_alias definition for Starlarkification of python flags
aranguyen Dec 10, 2025
e9688b7
Merge branch 'main' into main
rickeylev Dec 14, 2025
15687f1
Merge branch 'bazel-contrib:main' into main
aranguyen Dec 18, 2025
5c6539e
fix error
aranguyen Dec 18, 2025
504ccf3
use starlark flags instead
aranguyen Dec 18, 2025
effbe4c
fix starlark label
aranguyen Dec 18, 2025
e7bc464
remove extra character
aranguyen Dec 18, 2025
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5 changes: 5 additions & 0 deletions MODULE.bazel
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Original file line number Diff line number Diff line change
Expand Up @@ -458,3 +458,8 @@ uv_dev = use_extension(
uv_dev.configure(
version = "0.6.2",
)

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")
Comment on lines +462 to +465
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@shayanhoshyari shayanhoshyari Jan 8, 2026 •
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will this work for users of rules_python too?
otherwise I think this issue still stands : #3252 (comment)

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@gregestren gregestren Jan 8, 2026 •
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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.

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@shayanhoshyari shayanhoshyari Jan 8, 2026 •
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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.

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@gregestren gregestren Jan 8, 2026

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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.

2 changes: 1 addition & 1 deletion tests/base_rules/py_executable_base_tests.bzl
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Original file line number Diff line number Diff line change
Expand Up @@ -93,7 +93,7 @@ def _test_basic_zip(name, config):
# platforms.
# 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",
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@shayanhoshyari shayanhoshyari Jan 10, 2026 •
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@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

labels.BUILD_PYTHON_ZIP: True,
"//command_line_option:cpu": "linux_x86_64",
"//command_line_option:crosstool_top": CROSSTOOL_TOP,
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2 changes: 1 addition & 1 deletion tests/config_settings/transition/multi_version_tests.bzl
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Expand Up @@ -92,7 +92,7 @@ def _setup_py_binary_windows(name, *, impl, build_python_zip):
target = name + "_subject",
impl = impl,
config_settings = {
"//command_line_option:build_python_zip": str(build_python_zip),
"//python/config_settings:build_python_zip": str(build_python_zip),
labels.BUILD_PYTHON_ZIP: build_python_zip,
"//command_line_option:extra_toolchains": CC_TOOLCHAIN,
"//command_line_option:platforms": str(platform_targets.WINDOWS_X86_64),
Expand Down