fix(pypi): skip a pip.parse version with no interpreter for the host

news/4184.fixed.md

@@ -0,0 +1,6 @@
+(pypi) A {obj}`pip.parse` call for a Python version that has no interpreter for
+the host platform is now skipped, like a version with no registered toolchain,
+instead of failing the whole {obj}`pip` extension. A dependency that lists,
+say, Python 3.9 no longer breaks every other hub on hosts that have no CPython
+3.9 build, such as Windows on ARM64
+([#4184](https://github.com/bazel-contrib/rules_python/issues/4184)).

python/private/pypi/hub_builder.bzl

@@ -179,6 +179,25 @@ def _pip_parse(self, module_ctx, pip_attr, python_version = None):
         ))
         return
 
+    if (pip_attr.python_interpreter_target == None and
+        not pip_attr.python_interpreter and
+        _host_interpreter_name(python_version) not in self._available_interpreters):
+        # The version has a toolchain, but none of its interpreters runs on
+        # this host, e.g. there is no CPython build for this os and cpu. Treat
+        # it like a version missing from minor_mapping instead of failing the
+        # extension, which would also take down every other version and hub,
+        # including ones this host can use.
+        self._logger.info(lambda: (
+            "Ignoring pip python version '{version}' for hub " +
+            "'{hub}' in module '{module}' because there is no " +
+            "interpreter for it that runs on this host."
+        ).format(
+            hub = self.name,
+            module = self.module_name,
+            version = python_version,
+        ))
+        return
+
     _set_get_index_urls(self, module_ctx, pip_attr)
     self._platforms[python_version] = _platforms(
         module_ctx,
@@ -395,12 +414,13 @@ def _set_get_index_urls(self, mctx, pip_attr):
     )
     return True
 
+def _host_interpreter_name(python_version):
+    return "python_{}_host".format(python_version.replace(".", "_"))
+
 def _detect_interpreter(self, pip_attr, python_version):
     python_interpreter_target = pip_attr.python_interpreter_target
     if python_interpreter_target == None and not pip_attr.python_interpreter:
-        python_name = "python_{}_host".format(
-            python_version.replace(".", "_"),
-        )
+        python_name = _host_interpreter_name(python_version)
         if python_name not in self._available_interpreters:
             fail((
                 "Unable to find interpreter for pip hub '{hub_name}' for " +

tests/pypi/hub_builder/hub_builder_tests.bzl

@@ -529,6 +529,75 @@ new-package==0.0.1 --hash=sha256:deadb00f2
 
 _tests.append(_test_simple_multiple_python_versions)
 
+def _test_skip_python_version_without_host_interpreter(env):
+    # A module can ask for a Python version that has a toolchain but no
+    # interpreter for this host, e.g. a dependency listing 3.9 when rules_python
+    # ships no 3.9 CPython for aarch64-pc-windows-msvc. That version must be
+    # skipped instead of failing the whole extension, like a version missing
+    # from minor_mapping is.
+    builder = hub_builder(
+        env,
+        available_interpreters = {
+            "python_3_15_host": "unit_test_interpreter_target",
+        },
+        minor_mapping = {
+            "3.15": "3.15.19",
+            "3.16": "3.16.9",
+        },
+    )
+    builder.pip_parse(
+        mocks.mctx(
+            mock_files = {
+                "requirements_3_15.txt": """
+simple==0.0.1 --hash=sha256:deadbeef
+""",
+            },
+            os_name = "linux",
+            arch_name = "amd64",
+        ),
+        _parse(
+            hub_name = "pypi",
+            python_version = "3.15",
+            requirements_lock = "requirements_3_15.txt",
+        ),
+    )
+    builder.pip_parse(
+        mocks.mctx(
+            mock_files = {
+                "requirements_3_16.txt": """
+simple==0.0.2 --hash=sha256:deadb00f
+""",
+            },
+            os_name = "linux",
+            arch_name = "amd64",
+        ),
+        _parse(
+            hub_name = "pypi",
+            python_version = "3.16",
+            requirements_lock = "requirements_3_16.txt",
+        ),
+    )
+    pypi = builder.build()
+
+    pypi.exposed_packages().contains_exactly(["simple"])
+    pypi.whl_map().contains_exactly({
+        "simple": {
+            "pypi_315_simple": [
+                whl_config_setting(version = "3.15"),
+            ],
+        },
+    })
+    pypi.whl_libraries().contains_exactly({
+        "pypi_315_simple": {
+            "config_load": "@pypi//:config.bzl",
+            "dep_template": "@pypi//{name}:{target}",
+            "python_interpreter_target": "unit_test_interpreter_target",
+            "requirement": "simple==0.0.1 --hash=sha256:deadbeef",
+        },
+    })
+
+_tests.append(_test_skip_python_version_without_host_interpreter)
+
 def _test_simple_with_markers(env):
     sub_tests = {
         ("osx", "aarch64"): "torch==2.4.1 --hash=sha256:deadbeef",