add a better failure message and add compatibility aliases

Author: aignas

python/private/config_settings.bzl

@@ -68,7 +68,7 @@ def construct_config_settings(*, name, default_version, versions, minor_mapping,
         visibility = ["//visibility:public"],
     )
 
-    _reverse_minor_mapping = {full: minor for minor, full in minor_mapping.items()}
+    _reverse_minor_mapping = {full: minor for minor, full in minor_mapping.items() if full}
     for version in versions:
         minor_version = _reverse_minor_mapping.get(version)
         if not minor_version:
@@ -109,12 +109,19 @@ 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.
-    for minor in minor_mapping.keys():
-        native.config_setting(
-            name = "is_python_{}".format(minor),
-            flag_values = {_PYTHON_VERSION_MAJOR_MINOR_FLAG: minor},
-            visibility = ["//visibility:public"],
-        )
+    for minor, version in minor_mapping.items():
+        if version:
+            native.config_setting(
+                name = "is_python_{}".format(minor),
+                flag_values = {_PYTHON_VERSION_MAJOR_MINOR_FLAG: minor},
+                visibility = ["//visibility:public"],
+            )
+        else:
+            native.alias(
+                name = "is_python_{}".format(minor),
+                actual = "@platforms//:incompatible",
+                visibility = ["//visibility:public"],
+            )
 
     _current_config(
         name = "current_config",

python/private/full_version.bzl

@@ -26,6 +26,9 @@ def full_version(*, version, minor_mapping):
         major version then we return it as is.
     """
     if version in minor_mapping:
+        if not minor_mapping[version]:
+            fail("Please upgrade to the next version, '{}' is no longer supported".format(version))
+
         return minor_mapping[version]
 
     parts = version.split(".")

python/versions.bzl

@@ -1000,6 +1000,7 @@ TOOL_VERSIONS = {
 
 # buildifier: disable=unsorted-dict-items
 MINOR_MAPPING = {
+    "3.8": "",
     "3.9": "3.9.24",
     "3.10": "3.10.19",
     "3.11": "3.11.14",