extra fixes and add a helper for python_tag

Author: aignas

python/private/pypi/BUILD.bazel

@@ -116,6 +116,7 @@ bzl_library(
         ":parse_whl_name_bzl",
         ":pep508_env_bzl",
         ":pip_repository_attrs_bzl",
+        ":python_tag_bzl",
         ":simpleapi_download_bzl",
         ":whl_config_setting_bzl",
         ":whl_library_bzl",
@@ -337,6 +338,11 @@ bzl_library(
     ],
 )
 
+bzl_library(
+    name = "python_tag_bzl",
+    srcs = ["python_tag.bzl"],
+)
+
 bzl_library(
     name = "render_pkg_aliases_bzl",
     srcs = ["render_pkg_aliases.bzl"],
@@ -363,6 +369,7 @@ bzl_library(
     srcs = ["select_whl.bzl"],
     deps = [
         ":parse_whl_name_bzl",
+        ":python_tag_bzl",
         "//python/private:version_bzl",
     ],
 )

python/private/pypi/extension.bzl

@@ -31,6 +31,7 @@ load(":parse_requirements.bzl", "parse_requirements")
 load(":parse_whl_name.bzl", "parse_whl_name")
 load(":pep508_env.bzl", "env")
 load(":pip_repository_attrs.bzl", "ATTRS")
+load(":python_tag.bzl", "python_tag")
 load(":requirements_files_by_platform.bzl", "requirements_files_by_platform")
 load(":simpleapi_download.bzl", "simpleapi_download")
 load(":whl_config_setting.bzl", "whl_config_setting")
@@ -77,8 +78,7 @@ def _platforms(*, python_version, minor_mapping, config):
     )
 
     for platform, values in config.platforms.items():
-        # TODO @aignas 2025-07-07: fix the logic here
-        implementation = values.env["implementation_name"][:2].lower()
+        implementation = python_tag(values.env["implementation_name"])
 
         # TODO @aignas 2025-07-07: move the abi construction somewhere else
         abi = "{impl}{0}{1}.{2}".format(

python/private/pypi/pep508_env.bzl

@@ -67,16 +67,16 @@ platform_machine_select_map = {
 # Platform system returns results from the `uname` call.
 platform_system_select_map = {
     # See https://peps.python.org/pep-0738/#platform
-    "android": "Android",
-    "freebsd": "FreeBSD",
+    "@platforms//os:android": "Android",
+    "@platforms//os:freebsd": "FreeBSD",
     # See https://peps.python.org/pep-0730/#platform
     # NOTE: Per Pep 730, "iPadOS" is also an acceptable value
-    "ios": "iOS",
-    "linux": "Linux",
-    "netbsd": "NetBSD",
-    "openbsd": "OpenBSD",
-    "osx": "Darwin",
-    "windows": "Windows",
+    "@platforms//os:ios": "iOS",
+    "@platforms//os:linux": "Linux",
+    "@platforms//os:netbsd": "NetBSD",
+    "@platforms//os:openbsd": "OpenBSD",
+    "@platforms//os:osx": "Darwin",
+    "@platforms//os:windows": "Windows",
     # The value is empty string if it cannot be determined:
     # https://docs.python.org/3/library/platform.html#platform.machine
     _DEFAULT: "",

python/private/pypi/python_tag.bzl

@@ -0,0 +1,23 @@
+"A simple utility function to get the python_tag from the implementation name"
+
+# Taken from
+# https://packaging.python.org/en/latest/specifications/platform-compatibility-tags/#python-tag
+_PY_TAGS = {
+    # "py": Generic Python (does not require implementation-specific features)
+    "cpython": "cp",
+    "ironpython": "ip",
+    "jython": "jy",
+    "pypy": "pp",
+}
+PY_TAG_GENERIC = "py"
+
+def python_tag(implementation_name):
+    """Get the python_tag from the implementation_name.
+
+    Args:
+        implementation_name: {type}`str` the implementation name, e.g. "cpython"
+
+    Returns:
+        A {type}`str` that represents the python_tag.
+    """
+    return _PY_TAGS.get(implementation_name, implementation_name)

python/private/pypi/select_whl.bzl

@@ -2,17 +2,7 @@
 
 load("//python/private:version.bzl", "version")
 load(":parse_whl_name.bzl", "parse_whl_name")
-
-# Taken from
-# https://packaging.python.org/en/latest/specifications/platform-compatibility-tags/#python-tag
-_PY_TAGS = {
-    # "py": Generic Python (does not require implementation-specific features)
-    "cpython": "cp",
-    "ironpython": "ip",
-    "jython": "jy",
-    "pypy": "pp",
-}
-_PY = "py"
+load(":python_tag.bzl", "PY_TAG_GENERIC", "python_tag")
 
 def _get_priority(*, tag, values, allow_wildcard = True):
     for priority, wp in enumerate(values):
@@ -51,12 +41,12 @@ def select_whl(*, whls, python_version, platforms, want_abis, implementation_nam
     """
     py_version = version.parse(python_version, strict = True)
     candidates = {}
-    implementation = _PY_TAGS.get(implementation_name, implementation_name)
+    implementation = python_tag(implementation_name)
 
     for whl in whls:
         parsed = parse_whl_name(whl.filename)
 
-        if parsed.python_tag.startswith(_PY):
+        if parsed.python_tag.startswith(PY_TAG_GENERIC):
             pass
         elif not parsed.python_tag.startswith(implementation):
             if logger: