add warnings for the python toolchains that have been dropped

Author: aignas

python/private/config_settings.bzl

@@ -35,7 +35,13 @@ If the value is missing, then the default value is being used, see documentation
 # access it, but it's not intended for general public usage.
 _NOT_ACTUALLY_PUBLIC = ["//visibility:public"]
 
-def construct_config_settings(*, name, default_version, versions, minor_mapping, documented_flags):  # buildifier: disable=function-docstring
+def construct_config_settings(
+        *,
+        name,
+        default_version,
+        versions,
+        minor_mapping,
+        documented_flags):  # buildifier: disable=function-docstring
     """Create a 'python_version' config flag and construct all config settings used in rules_python.
 
     This mainly includes the targets that are used in the toolchain and pip hub
@@ -109,13 +115,29 @@ def construct_config_settings(*, name, default_version, versions, minor_mapping,
     # It's private because matching the concept of e.g. "3.8" value is done
     # using the `is_python_X.Y` config setting group, which is aware of the
     # minor versions that could match instead.
+    first_minor = None
     for minor in minor_mapping.keys():
+        if first_minor == None:
+            first_minor = minor
+
         native.config_setting(
             name = "is_python_{}".format(minor),
             flag_values = {_PYTHON_VERSION_MAJOR_MINOR_FLAG: minor},
             visibility = ["//visibility:public"],
         )
 
+    for minor in range(int(first_minor.partition(".")[-1]) + 1):
+        minor = "3.{}".format(minor)
+        if minor in minor_mapping:
+            break
+
+        # TODO @aignas 2025-11-04: use env-marker-setting with the smallest minor_mapping version
+        native.alias(
+            name = "is_python_{}".format(minor),
+            actual = "@platforms//:incompatible",
+            visibility = ["//visibility:public"],
+        )
+
     _current_config(
         name = "current_config",
         build_setting_default = "",

python/private/full_version.bzl

@@ -14,12 +14,13 @@
 
 """A small helper to ensure that we are working with full versions."""
 
-def full_version(*, version, minor_mapping):
+def full_version(*, version, minor_mapping, err = True):
     """Return a full version.
 
     Args:
         version: {type}`str` the version in `X.Y` or `X.Y.Z` format.
         minor_mapping: {type}`dict[str, str]` mapping between `X.Y` to `X.Y.Z` format.
+        err: {type}`bool` whether to fail on error or return `None` instead.
 
     Returns:
         a full version given the version string. If the string is already a
@@ -31,6 +32,8 @@ def full_version(*, version, minor_mapping):
     parts = version.split(".")
     if len(parts) == 3:
         return version
+    elif not err:
+        return None
     elif len(parts) == 2:
         fail(
             "Unknown Python version '{}', available values are: {}".format(

python/private/pypi/hub_builder.bzl

@@ -114,9 +114,25 @@ def _pip_parse(self, module_ctx, pip_attr):
             version = python_version,
         ))
 
-    self._platforms[python_version] = _platforms(
-        python_version = python_version,
+    full_python_version = full_version(
+        version = python_version,
         minor_mapping = self._minor_mapping,
+        err = False,
+    )
+    if not full_python_version:
+        self._logger.warn(lambda: (
+            "Ignoring pip python version '{version}' for hub " +
+            "'{hub}' in module '{module}' because there is no registered " +
+            "toolchain for it."
+        ).format(
+            hub = self.name,
+            module = self.module_name,
+            version = python_version,
+        ))
+        return
+
+    self._platforms[python_version] = _platforms(
+        python_version = full_python_version,
         config = self._config,
     )
     _set_get_index_urls(self, pip_attr)
@@ -280,13 +296,10 @@ def _detect_interpreter(self, pip_attr):
         path = pip_attr.python_interpreter,
     )
 
-def _platforms(*, python_version, minor_mapping, config):
+def _platforms(*, python_version, config):
     platforms = {}
     python_version = version.parse(
-        full_version(
-            version = python_version,
-            minor_mapping = minor_mapping,
-        ),
+        python_version,
         strict = True,
     )
 

python/private/python.bzl

@@ -268,7 +268,12 @@ def _python_impl(module_ctx):
         full_python_version = full_version(
             version = toolchain_info.python_version,
             minor_mapping = py.config.minor_mapping,
+            err = False,
         )
+        if not full_python_version:
+            logger.warn(lambda: "The python version '{}' is unknown, please configure a toolchain to be downloaded".format(toolchain_info.python_version))
+            continue
+
         kwargs = {
             "python_version": full_python_version,
             "register_coverage_tool": toolchain_info.register_coverage_tool,