fix(pip): set better defaults for the new target_platforms attr (#3447)

Fixup the default values for the `target_platforms` so that the
users can actually switch it as per docs.

I have also taken the liberty to update all of the tests to better
reflect how we set things up for legacy and index-url setups. As
it is now, legacy `bzlmod` whl_library layout is much more similar
to WORKSPACE, which makes the transition easier.

Work towards #2949

Author: aignas

MODULE.bazel

@@ -305,6 +305,11 @@ dev_pip = use_extension(
         parallel_download = False,
         python_version = python_version,
         requirements_lock = "//docs:requirements.txt",
+        # Ensure that we are setting up the following platforms
+        target_platforms = [
+            "{os}_{arch}",
+            "{os}_{arch}_freethreaded",
+        ],
     )
     for python_version in [
         "3.9",

docs/BUILD.bazel

@@ -165,6 +165,8 @@ sphinx_build_binary(
     config_settings = {
         labels.BOOTSTRAP_IMPL: "script",
         labels.VENVS_SITE_PACKAGES: "yes",
+        labels.PY_FREETHREADED: "yes",
+        labels.PYTHON_VERSION: "3.14",
     },
     target_compatible_with = _TARGET_COMPATIBLE_WITH,
     deps = [

python/private/pypi/extension.bzl

@@ -665,7 +665,7 @@ EXPERIMENTAL: this may be removed without notice.
 """,
         ),
         "target_platforms": attr.string_list(
-            default = ["{os}_{arch}"],
+            default = [],
             doc = """\
 The list of platforms for which we would evaluate the requirements files. If you need to be able to
 only evaluate for a particular platform (e.g. "linux_x86_64"), then put it in here.

python/private/pypi/hub_builder.bzl

@@ -141,9 +141,10 @@ def _pip_parse(self, module_ctx, pip_attr):
         module_ctx,
         python_version = full_python_version,
         config = self._config,
-        # FIXME @aignas 2025-12-06: should we have this behaviour?
-        # TODO @aignas 2025-12-06: use target_platforms always even when the get_index_urls is set.
-        target_platforms = [] if default_cross_setup else pip_attr.target_platforms,
+        # TODO @aignas 2025-12-09: flip or part to default to 'os_arch' after
+        # VERSION_NEXT_FEATURE is released and set the default of the `target_platforms` attribute
+        # to `{os}_{arch}`.
+        target_platforms = pip_attr.target_platforms or ([] if default_cross_setup else ["{os}_{arch}"]),
     )
     _add_group_map(self, pip_attr.experimental_requirement_cycles)
     _add_extra_aliases(self, pip_attr.extra_hub_aliases)

python/private/pypi/requirements_files_by_platform.bzl

@@ -144,6 +144,8 @@ def requirements_files_by_platform(
 
     input_platforms = platforms
     default_platforms = [_platform(p, python_version) for p in platforms]
+    if logger:
+        logger.debug(lambda: "Input platforms: {}".format(input_platforms))
 
     if platforms_from_args:
         lock_files = [

tests/pypi/extension/extension_tests.bzl

@@ -22,12 +22,12 @@ load(":pip_parse.bzl", _parse = "pip_parse")
 
 _tests = []
 
-def _mock_mctx(*modules, environ = {}, read = None):
+def _mock_mctx(*modules, os_name = "unittest", arch_name = "exotic", environ = {}, read = None):
     return struct(
         os = struct(
             environ = environ,
-            name = "unittest",
-            arch = "exotic",
+            name = os_name,
+            arch = arch_name,
         ),
         read = read or (lambda _: """\
 simple==0.0.1 \
@@ -148,6 +148,8 @@ def _test_simple(env):
                     ),
                 ],
             ),
+            os_name = "linux",
+            arch_name = "x86_64",
         ),
         available_interpreters = {
             "python_3_15_host": "unit_test_interpreter_target",