internal(render_pkg_aliases): allow using default config setting

python/private/bzlmod/pip.bzl

@@ -208,6 +208,7 @@ def _create_whl_repos(module_ctx, pip_attr, whl_map, whl_overrides):
         )
 
         major_minor = _major_minor_version(pip_attr.python_version)
+        default_major_minor = _major_minor_version(DEFAULT_PYTHON_VERSION)
         whl_map[hub_name].setdefault(whl_name, []).append(
             whl_alias(
                 repo = repo_name,
@@ -216,6 +217,14 @@ def _create_whl_repos(module_ctx, pip_attr, whl_map, whl_overrides):
                 config_setting = Label("//python/config_settings:is_python_" + major_minor),
             ),
         )
+        if default_major_minor == major_minor:
+            whl_map[hub_name][whl_name].append(
+                whl_alias(
+                    repo = repo_name,
+                    version = major_minor,
+                    config_setting = "//conditions:default",
+                ),
+            )
 
 def _pip_impl(module_ctx):
     """Implementation of a class tag that creates the pip hub and corresponding pip spoke whl repositories.

python/private/bzlmod/pip_repository.bzl

@@ -31,7 +31,6 @@ def _pip_repository_impl(rctx):
             key: [whl_alias(**v) for v in json.decode(values)]
             for key, values in rctx.attr.whl_map.items()
         },
-        default_version = rctx.attr.default_version,
     )
     for path, contents in aliases.items():
         rctx.file(path, contents)

python/private/render_pkg_aliases.bzl

@@ -38,6 +38,8 @@ If the value is missing, then the "default" Python version is being used,
 which has a "null" version value and will not match version constraints.
 """
 
+_CONDITIONS_DEFAULT = "//conditions:default"
+
 def _render_whl_library_alias(
         *,
         name,
@@ -60,12 +62,13 @@ def _render_whl_library_alias(
     for alias in sorted(aliases, key = lambda x: x.version):
         actual = "@{repo}//:{name}".format(repo = alias.repo, name = name)
         selects[alias.config_setting] = actual
-        if alias.version == default_version:
+        if alias.version == default_version or alias.config_setting == _CONDITIONS_DEFAULT:
             default = actual
             no_match_error = None
 
     if default:
-        selects["//conditions:default"] = default
+        # Attempt setting it but continue if already exists
+        selects.setdefault(_CONDITIONS_DEFAULT, default)
 
     return render.alias(
         name = name,
@@ -81,10 +84,24 @@ def _render_common_aliases(*, name, aliases, default_version = None):
     ]
 
     versions = None
+    has_default = False
     if aliases:
-        versions = sorted([v.version for v in aliases if v.version])
+        versions = sorted([a.version for a in aliases if a.version])
+        has_default = len([a.config_setting for a in aliases if a.config_setting == _CONDITIONS_DEFAULT]) == 1
+        default_version_aliases = [a for a in aliases if a.version == default_version or a.config_setting == _CONDITIONS_DEFAULT]
+        if not has_default and len(default_version_aliases) > 1:
+            fail(
+                (
+                    "BUG: expected to have a single alias for the default version, but got multiple: '{}'. " +
+                    "Add the 'whl_alias(config_setting = {}, ...)' setting explicitly."
+                ).format(default_version_aliases, repr(_CONDITIONS_DEFAULT)),
+            )
+
+        if has_default:
+            # Zero this out as it is not useful anymore.
+            default_version = None
 
-    if not versions or default_version in versions:
+    if not versions or default_version in versions or has_default:
         pass
     else:
         error_msg = NO_MATCH_ERROR_MESSAGE_TEMPLATE.format(

tests/pip_hub_repository/render_pkg_aliases/render_pkg_aliases_test.bzl

@@ -160,6 +160,75 @@ alias(
 
 _tests.append(_test_bzlmod_aliases)
 
+def _test_bzlmod_aliases_with_default(env):
+    actual = render_pkg_aliases(
+        aliases = {
+            "bar-baz": [
+                whl_alias(version = "3.2", repo = "pypi_32_bar_baz", config_setting = "//:my_config_setting_32"),
+                whl_alias(version = "3.1", repo = "pypi_31_bar_baz", config_setting = "//:my_config_setting_31"),
+                whl_alias(version = "3.2", repo = "pypi_32_bar_baz", config_setting = "//conditions:default"),
+            ],
+        },
+    )
+
+    want_key = "bar_baz/BUILD.bazel"
+    want_content = """\
+package(default_visibility = ["//visibility:public"])
+
+alias(
+    name = "bar_baz",
+    actual = ":pkg",
+)
+
+alias(
+    name = "pkg",
+    actual = select(
+        {
+            "//:my_config_setting_31": "@pypi_31_bar_baz//:pkg",
+            "//:my_config_setting_32": "@pypi_32_bar_baz//:pkg",
+            "//conditions:default": "@pypi_32_bar_baz//:pkg",
+        },
+    ),
+)
+
+alias(
+    name = "whl",
+    actual = select(
+        {
+            "//:my_config_setting_31": "@pypi_31_bar_baz//:whl",
+            "//:my_config_setting_32": "@pypi_32_bar_baz//:whl",
+            "//conditions:default": "@pypi_32_bar_baz//:whl",
+        },
+    ),
+)
+
+alias(
+    name = "data",
+    actual = select(
+        {
+            "//:my_config_setting_31": "@pypi_31_bar_baz//:data",
+            "//:my_config_setting_32": "@pypi_32_bar_baz//:data",
+            "//conditions:default": "@pypi_32_bar_baz//:data",
+        },
+    ),
+)
+
+alias(
+    name = "dist_info",
+    actual = select(
+        {
+            "//:my_config_setting_31": "@pypi_31_bar_baz//:dist_info",
+            "//:my_config_setting_32": "@pypi_32_bar_baz//:dist_info",
+            "//conditions:default": "@pypi_32_bar_baz//:dist_info",
+        },
+    ),
+)"""
+
+    env.expect.that_collection(actual.keys()).contains_exactly([want_key])
+    env.expect.that_str(actual[want_key]).equals(want_content)
+
+_tests.append(_test_bzlmod_aliases_with_default)
+
 def _test_bzlmod_aliases_with_no_default_version(env):
     actual = render_pkg_aliases(
         default_version = None,