fix(bzlmod): set the default_version to the python_version flag (bazel-contrib#2253)

With this change we set the default value of `--python_version` when
the `python.toolchain` is used in `bzlmod` and we generate the
appropriate config settings based on the registered toolchains and
given overrides by the root module.

This means that we expect the `--python_version` to be always set to
a non-empty value under `bzlmod` and we can cleanup code which was
handling `//conditions:default` case. This also means that we can
in theory drop the requirement for `python_version` in `pip.parse`
and just setup dependencies for all packages that we find in the
`requirements.txt` file and move on. This is left as future work
by myself or anyone willing to contribute. We can also start reusing
the same `whl_library` instance for multi-platform packages because
the python version flag is always set - this will simplify the layout
and makes the feature non-experimental anymore under bzlmod.

Summary:
* Add `@pythons_hub` to the `WORKSPACE` with dummy data to make
  pythons_hub work.
* Add `MINOR_MAPPING` and `PYTHON_VERSIONS` to pythons_hub to
  generate the config settings.
* Remove handling of the default version in `pypi` code under bzlmod.

Work towards bazel-contrib#2081, bazel-contrib#260, bazel-contrib#1708

---------

Co-authored-by: Richard Levasseur <[email protected]>

Author: aignas

CHANGELOG.md

@@ -29,6 +29,11 @@ A brief description of the categories of changes:
   (previously it defaulted to None).
 
 ### Fixed
+* (bzlmod) The `python.override(minor_mapping)` now merges the default and the
+  overridden versions ensuring that the resultant `minor_mapping` will always
+  have all of the python versions.
+* (bzlmod) The default value for the {obj}`--python_version` flag will now be
+  always set to the default python toolchain version value.
 * (bzlmod) correctly wire the {attr}`pip.parse.extra_pip_args` all the
   way to {obj}`whl_library`. What is more we will pass the `extra_pip_args` to
   {obj}`whl_library` for `sdist` distributions when using

WORKSPACE

@@ -41,14 +41,14 @@ load("//:internal_setup.bzl", "rules_python_internal_setup")
 
 rules_python_internal_setup()
 
+load("@pythons_hub//:versions.bzl", "MINOR_MAPPING", "PYTHON_VERSIONS")
 load("//python:repositories.bzl", "python_register_multi_toolchains")
-load("//python:versions.bzl", "MINOR_MAPPING", "TOOL_VERSIONS")
 
 python_register_multi_toolchains(
     name = "python",
     default_version = MINOR_MAPPING.values()[-2],
     # Integration tests verify each version, so register all of them.
-    python_versions = TOOL_VERSIONS.keys(),
+    python_versions = PYTHON_VERSIONS,
 )
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive", "http_file")

examples/bzlmod/MODULE.bazel

@@ -42,17 +42,16 @@ python.toolchain(
 python.override(
     available_python_versions = [
         "3.10.9",
+        "3.9.18",
         "3.9.19",
         # The following is used by the `other_module` and we need to include it here
         # as well.
         "3.11.8",
     ],
     # Also override the `minor_mapping` so that the root module,
-    # instead of rules_python's defaults, controls what full version
-    # is used when `3.x` is requested.
+    # instead of rules_python's defaulting to the latest available version,
+    # controls what full version is used when `3.x` is requested.
     minor_mapping = {
-        "3.10": "3.10.9",
-        "3.11": "3.11.8",
         "3.9": "3.9.19",
     },
 )
@@ -90,7 +89,7 @@ python.single_version_platform_override(
 # See the tests folder for various examples on using multiple Python versions.
 # The names "python_3_9" and "python_3_10" are autmatically created by the repo
 # rules based on the `python_version` arg values.
-use_repo(python, "python_3_10", "python_3_9", "python_versions")
+use_repo(python, "python_3_10", "python_3_9", "python_versions", "pythons_hub")
 
 # EXPERIMENTAL: This is experimental and may be removed without notice
 uv = use_extension("@rules_python//python/uv:extensions.bzl", "uv")

examples/bzlmod/MODULE.bazel.lock

@@ -1,8 +1,8 @@
 load("@python_versions//3.10:defs.bzl", py_binary_3_10 = "py_binary", py_test_3_10 = "py_test")
 load("@python_versions//3.11:defs.bzl", py_binary_3_11 = "py_binary", py_test_3_11 = "py_test")
 load("@python_versions//3.9:defs.bzl", py_binary_3_9 = "py_binary", py_test_3_9 = "py_test")
+load("@pythons_hub//:versions.bzl", "MINOR_MAPPING")
 load("@rules_python//python:defs.bzl", "py_binary", "py_test")
-load("@rules_python//python:versions.bzl", "MINOR_MAPPING")
 load("@rules_python//python/config_settings:transition.bzl", py_versioned_binary = "py_binary", py_versioned_test = "py_test")
 
 py_binary(

examples/bzlmod/tests/BUILD.bazel

@@ -22,13 +22,25 @@ load("@rules_bazel_integration_test//bazel_integration_test:deps.bzl", "bazel_in
 load("@rules_bazel_integration_test//bazel_integration_test:repo_defs.bzl", "bazel_binaries")
 load("@rules_proto//proto:repositories.bzl", "rules_proto_dependencies", "rules_proto_toolchains")
 load("//:version.bzl", "SUPPORTED_BAZEL_VERSIONS")
+load("//python:versions.bzl", "MINOR_MAPPING", "TOOL_VERSIONS")
 load("//python/private:internal_config_repo.bzl", "internal_config_repo")  # buildifier: disable=bzl-visibility
+load("//python/private:pythons_hub.bzl", "hub_repo")  # buildifier: disable=bzl-visibility
 load("//python/private/pypi:deps.bzl", "pypi_deps")  # buildifier: disable=bzl-visibility
 
 def rules_python_internal_setup():
     """Setup for rules_python tests and tools."""
 
     internal_config_repo(name = "rules_python_internal")
+    hub_repo(
+        name = "pythons_hub",
+        minor_mapping = MINOR_MAPPING,
+        default_python_version = "",
+        toolchain_prefixes = [],
+        toolchain_python_versions = [],
+        toolchain_set_python_version_constraints = [],
+        toolchain_user_repository_names = [],
+        python_versions = sorted(TOOL_VERSIONS.keys()),
+    )
 
     # Because we don't use the pip_install rule, we have to call this to fetch its deps
     pypi_deps()

internal_setup.bzl

@@ -1,5 +1,5 @@
 load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
-load("//python:versions.bzl", "MINOR_MAPPING", "TOOL_VERSIONS")
+load("@pythons_hub//:versions.bzl", "DEFAULT_PYTHON_VERSION", "MINOR_MAPPING", "PYTHON_VERSIONS")
 load(
     "//python/private:flags.bzl",
     "BootstrapImplFlag",
@@ -28,8 +28,9 @@ filegroup(
 
 construct_config_settings(
     name = "construct_config_settings",
+    default_version = DEFAULT_PYTHON_VERSION,
     minor_mapping = MINOR_MAPPING,
-    versions = TOOL_VERSIONS.keys(),
+    versions = PYTHON_VERSIONS,
 )
 
 string_flag(

python/config_settings/BUILD.bazel

@@ -165,6 +165,8 @@ bzl_library(
     deps = [
         ":bazel_tools_bzl",
         ":internal_config_repo_bzl",
+        ":pythons_hub_bzl",
+        "//python:versions_bzl",
         "//python/private/pypi:deps_bzl",
     ],
 )
@@ -212,6 +214,7 @@ bzl_library(
     srcs = ["pythons_hub.bzl"],
     deps = [
         ":py_toolchain_suite_bzl",
+        ":text_util_bzl",
     ],
 )
 

python/private/BUILD.bazel

@@ -22,25 +22,22 @@ load(":semver.bzl", "semver")
 _PYTHON_VERSION_FLAG = Label("//python/config_settings:python_version")
 _PYTHON_VERSION_MAJOR_MINOR_FLAG = Label("//python/config_settings:_python_version_major_minor")
 
-def construct_config_settings(*, name, versions, minor_mapping):  # buildifier: disable=function-docstring
+def construct_config_settings(*, name, default_version, versions, minor_mapping):  # 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
     repositories that only match on the 'python_version' flag values.
 
     Args:
         name: {type}`str` A dummy name value that is no-op for now.
+        default_version: {type}`str` the default value for the `python_version` flag.
         versions: {type}`list[str]` A list of versions to build constraint settings for.
         minor_mapping: {type}`dict[str, str]` A mapping from `X.Y` to `X.Y.Z` python versions.
     """
     _ = name  # @unused
     _python_version_flag(
         name = _PYTHON_VERSION_FLAG.name,
-        # TODO: The default here should somehow match the MODULE config. Until
-        # then, use the empty string to indicate an unknown version. This
-        # also prevents version-unaware targets from inadvertently matching
-        # a select condition when they shouldn't.
-        build_setting_default = "",
+        build_setting_default = default_version,
         visibility = ["//visibility:public"],
     )
 

python/private/config_settings.bzl

@@ -16,8 +16,10 @@
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", _http_archive = "http_archive")
 load("@bazel_tools//tools/build_defs/repo:utils.bzl", "maybe")
+load("//python:versions.bzl", "MINOR_MAPPING", "TOOL_VERSIONS")
 load("//python/private/pypi:deps.bzl", "pypi_deps")
 load(":internal_config_repo.bzl", "internal_config_repo")
+load(":pythons_hub.bzl", "hub_repo")
 
 def http_archive(**kwargs):
     maybe(_http_archive, **kwargs)
@@ -32,6 +34,17 @@ def py_repositories():
         internal_config_repo,
         name = "rules_python_internal",
     )
+    maybe(
+        hub_repo,
+        name = "pythons_hub",
+        minor_mapping = MINOR_MAPPING,
+        default_python_version = "",
+        toolchain_prefixes = [],
+        toolchain_python_versions = [],
+        toolchain_set_python_version_constraints = [],
+        toolchain_user_repository_names = [],
+        python_versions = sorted(TOOL_VERSIONS.keys()),
+    )
     http_archive(
         name = "bazel_skylib",
         sha256 = "74d544d96f4a5bb630d465ca8bbcfe231e3594e5aae57e1edbf17a6eb3ca2506",