enable_pipstar=True breaks extracting wheels when no python interpreter is present

[a-khabarov]

🐞 bug report

Affected Rule

The issue is caused by the rule:

pip_parse

Is this a regression?

Yes, the bug was not present in 1.7.0. It is present in 1.8.0 and 1.9.0.
89fedb7 is the commit that caused this breakage.

Description

rules_python 1.8.0+ with bootstrap_impl=script and experimental_index_url can attempt to invoke the system python interpreter, causing a failure in any environment that doesn't have a system python installed.

🔬 Minimal Reproduction

.bazelrc

common --@rules_python//python/config_settings:bootstrap_impl=script

.bazelversion

8.4.2

BUILD.bazel
(empty)

MODULE.bazel

module(name = "pre-commit")

bazel_dep(name = "rules_python", version = "1.9.0")
bazel_dep(name = "rules_uv", version = "0.56.0", dev_dependency = True)
python = use_extension("@rules_python//python/extensions:python.bzl", "python")
pip = use_extension("@rules_python//python/extensions:pip.bzl", "pip")

python.toolchain(python_version = "3.13")
pip.parse(
    experimental_index_url = "https://pypi.org/simple",
    hub_name = "pre-commit",
    python_version = "3.13",
    requirements_lock = "//pre-commit/requirements:lock.txt",
)

use_repo(pip, pkg = "pre-commit")

pre-commit/BUILD.bazel

alias(
    name = "pkg",
    actual = "@pkg//pre_commit:pkg",
    visibility = ["//visibility:public"],
)

pre-commit/requirements/BUILD.bazel

load("@rules_uv//uv:pip.bzl", "pip_compile")

pip_compile(
    name = "requirements",
    requirements_in = "in.txt",
    requirements_txt = "lock.txt",
    universal = True,
)

pre-commit/requirements/in.txt

pre-commit==4.0.1

Ensure that system python is not installed and that the python3 command is not available.
To reproduce the failure:

bazelisk run pre-commit/requirements
bazelisk build pre-commit:pkg

🔥 Exception or Error

Traceback (most recent call last):
        File ".../external/rules_python+/python/private/pypi/whl_library.bzl", line 302, column 68, in _whl_library_impl
                python_interpreter = pypi_repo_utils.resolve_python_interpreter(
        File ".../external/rules_python+/python/private/pypi/pypi_repo_utils.bzl", line 78, column 54, in _resolve_python_interpreter
                python_interpreter = repo_utils.which_checked(mrctx, python_interpreter)
        File ".../external/rules_python+/python/private/repo_utils.bzl", line 282, column 13, in _which_checked
                fail(result.describe_failure())
Error in fail: Unable to find the binary 'python3' on PATH.

🌍 Your Environment

Operating System:

  
Linux
  

Output of bazel version:

  
Bazelisk version: development
Build label: 8.4.2
Build target: @@//src/main/java/com/google/devtools/build/lib/bazel:BazelServer
Build time: Wed Oct 01 16:46:46 2025 (1759337206)
Build timestamp: 1759337206
Build timestamp as int: 1759337206
  

Rules_python version:

  
1.8.0 and 1.9.0
  

Anything else relevant?

The following patch makes the build pass:

diff --git a/python/private/pypi/hub_builder.bzl b/python/private/pypi/hub_builder.bzl
index f0aa6a73..6c5e28c8 100644
--- a/python/private/pypi/hub_builder.bzl
+++ b/python/private/pypi/hub_builder.bzl
@@ -619,6 +619,8 @@ def _create_whl_repos(
             )
 
 def _common_args(self, module_ctx, *, pip_attr, enable_pipstar):
+    interpreter = _detect_interpreter(self, pip_attr)
+
     # Construct args separately so that the lock file can be smaller and does not include unused
     # attrs.
     whl_library_args = dict(
@@ -633,6 +635,8 @@ def _common_args(self, module_ctx, *, pip_attr, enable_pipstar):
         environment = pip_attr.environment,
         envsubst = pip_attr.envsubst,
         pip_data_exclude = pip_attr.pip_data_exclude,
+        python_interpreter = interpreter.path,
+        python_interpreter_target = interpreter.target,
     )
     if not enable_pipstar:
         maybe_args["experimental_target_platforms"] = pip_attr.experimental_target_platforms

[aignas]

It seems that the whl_library code in some place is incorrectly using the python interpreter where it should not need to - we can extract wheels without any Python present on the system. Whilst the attached patch is good enough to unblock people, the correct fix is slightly different.

[a-khabarov]

@aignas I am still getting Unable to find the binary 'python3' on PATH. on 2.0.0-rc3, which, looking at 2.0.0-rc2...2.0.0-rc3, includes #3720. I think this should be reopened.

   Traceback (most recent call last):
        File "/home/.../external/rules_python+/python/private/pypi/whl_library.bzl", line 295, column 68, in _whl_library_impl
                python_interpreter = pypi_repo_utils.resolve_python_interpreter(
        File "/home/.../external/rules_python+/python/private/pypi/pypi_repo_utils.bzl", line 78, column 54, in _resolve_python_interpreter
                python_interpreter = repo_utils.which_checked(mrctx, python_interpreter)
        File "/home/.../external/rules_python+/python/private/repo_utils.bzl", line 282, column 13, in _which_checked
                fail(result.describe_failure())
Error in fail: Unable to find the binary 'python3' on PATH.

[aignas]

I think the following patch may work, please confirm:

index 5db4f12d..ec5921a7 100644
--- a/python/private/pypi/whl_library.bzl
+++ b/python/private/pypi/whl_library.bzl
@@ -183,7 +183,8 @@ def _parse_optional_attrs(rctx, args, extra_pip_args = None):
     if rctx.attr.add_libdir_to_library_search_path:
         if "LDFLAGS" in env:
             fail("Can't set both environment LDFLAGS and add_libdir_to_library_search_path")
-        command = [pypi_repo_utils.resolve_python_interpreter(rctx), "-c", "import sys ; sys.stdout.write('{}/lib'.format(sys.exec_prefix))"]
+        command = [
+            pypi_repo_utils.resolve_python_interpreter(rctx), "-c", "import sys ; sys.stdout.write('{}/lib'.format(sys.exec_prefix))"]
         result = rctx.execute(command)
         if result.return_code != 0:
             fail("Failed to get LDFLAGS path: command: {}, exit code: {}, stdout: {}, stderr: {}".format(command, result.return_code, result.stdout, result.stderr))
@@ -292,22 +293,11 @@ def _extract_whl_py(rctx, *, python_interpreter, args, whl_path, environment, lo
 
 def _whl_library_impl(rctx):
     logger = repo_utils.logger(rctx)
-    python_interpreter = pypi_repo_utils.resolve_python_interpreter(
-        rctx,
-        python_interpreter = rctx.attr.python_interpreter,
-        python_interpreter_target = rctx.attr.python_interpreter_target,
-    )
-    args = [
-        "-m",
-        "python.private.pypi.whl_installer.wheel_installer",
-        "--requirement",
-        rctx.attr.requirement,
-    ]
-    extra_pip_args = []
-    extra_pip_args.extend(rctx.attr.extra_pip_args)
 
     whl_path = None
     sdist_filename = None
+    extra_pip_args = []
+    extra_pip_args.extend(rctx.attr.extra_pip_args)
     if rctx.attr.whl_file:
         rctx.watch(rctx.attr.whl_file)
         whl_path = rctx.path(rctx.attr.whl_file)
@@ -352,8 +342,6 @@ def _whl_library_impl(rctx):
             # build deps from PyPI (e.g. `flit_core`) if they are missing.
             extra_pip_args.extend(["--find-links", "."])
 
-    args = _parse_optional_attrs(rctx, args, extra_pip_args)
-
     # also enable pipstar for any whls that are downloaded without `pip`
     enable_pipstar = (rp_config.enable_pipstar or whl_path) and rctx.attr.config_load
     enable_pipstar_extract = enable_pipstar and rp_config.bazel_8_or_later
@@ -362,7 +350,21 @@ def _whl_library_impl(rctx):
     # to setup env vars to run Python, unless we need to build an sdist
     if enable_pipstar_extract and whl_path:
         environment = {}
+        args = []
+        python_interpreter = None
     else:
+        python_interpreter = pypi_repo_utils.resolve_python_interpreter(
+            rctx,
+            python_interpreter = rctx.attr.python_interpreter,
+            python_interpreter_target = rctx.attr.python_interpreter_target,
+        )
+        args = [
+            "-m",
+            "python.private.pypi.whl_installer.wheel_installer",
+            "--requirement",
+            rctx.attr.requirement,
+        ]
+        args = _parse_optional_attrs(rctx, args, extra_pip_args)
         # Manually construct the PYTHONPATH since we cannot use the toolchain here
         environment = _create_repository_execution_environment(rctx, python_interpreter, logger = logger)
 

I missed the fact that were still resolving the python interpreter.

[a-khabarov]

This patch fixes the problem for me (with it applied on top of latest main, commit 1c8ca73).