fix: Don't require default Python version for pip hubs

.bazelrc

@@ -3,8 +3,8 @@
 # This lets us glob() up all the files inside the examples to make them inputs to tests
 # (Note, we cannot use `common --deleted_packages` because the bazel version command doesn't support it)
 # To update these lines, run tools/bazel_integration_test/update_deleted_packages.sh
-build --deleted_packages=examples/build_file_generation,examples/build_file_generation/random_number_generator,examples/bzlmod,examples/bzlmod/entry_point,examples/bzlmod/libs/my_lib,examples/bzlmod/other_module/other_module/pkg,examples/bzlmod/runfiles,examples/bzlmod/tests,examples/bzlmod/whl_mods,examples/bzlmod_build_file_generation,examples/bzlmod_build_file_generation/other_module/other_module/pkg,examples/bzlmod_build_file_generation/runfiles,examples/multi_python_versions/libs/my_lib,examples/multi_python_versions/requirements,examples/multi_python_versions/tests,examples/pip_install,examples/pip_parse,examples/pip_parse_vendored,examples/pip_repository_annotations,examples/py_proto_library,tests/compile_pip_requirements,tests/compile_pip_requirements_test_from_external_workspace,tests/ignore_root_user_error,tests/pip_repository_entry_points
-query --deleted_packages=examples/build_file_generation,examples/build_file_generation/random_number_generator,examples/bzlmod,examples/bzlmod/entry_point,examples/bzlmod/libs/my_lib,examples/bzlmod/other_module/other_module/pkg,examples/bzlmod/runfiles,examples/bzlmod/tests,examples/bzlmod/whl_mods,examples/bzlmod_build_file_generation,examples/bzlmod_build_file_generation/other_module/other_module/pkg,examples/bzlmod_build_file_generation/runfiles,examples/multi_python_versions/libs/my_lib,examples/multi_python_versions/requirements,examples/multi_python_versions/tests,examples/pip_install,examples/pip_parse,examples/pip_parse_vendored,examples/pip_repository_annotations,examples/py_proto_library,tests/compile_pip_requirements,tests/compile_pip_requirements_test_from_external_workspace,tests/ignore_root_user_error,tests/pip_repository_entry_points
+build --deleted_packages=examples/build_file_generation,examples/build_file_generation/random_number_generator,examples/bzlmod,examples/bzlmod_build_file_generation,examples/bzlmod_build_file_generation/other_module/other_module/pkg,examples/bzlmod_build_file_generation/runfiles,examples/bzlmod/entry_point,examples/bzlmod/libs/my_lib,examples/bzlmod/other_module,examples/bzlmod/other_module/other_module/pkg,examples/bzlmod/runfiles,examples/bzlmod/tests,examples/bzlmod/tests/other_module,examples/bzlmod/whl_mods,examples/multi_python_versions/libs/my_lib,examples/multi_python_versions/requirements,examples/multi_python_versions/tests,examples/pip_install,examples/pip_parse,examples/pip_parse_vendored,examples/pip_repository_annotations,examples/py_proto_library,tests/compile_pip_requirements,tests/compile_pip_requirements_test_from_external_workspace,tests/ignore_root_user_error,tests/pip_repository_entry_points
+query --deleted_packages=examples/build_file_generation,examples/build_file_generation/random_number_generator,examples/bzlmod,examples/bzlmod_build_file_generation,examples/bzlmod_build_file_generation/other_module/other_module/pkg,examples/bzlmod_build_file_generation/runfiles,examples/bzlmod/entry_point,examples/bzlmod/libs/my_lib,examples/bzlmod/other_module,examples/bzlmod/other_module/other_module/pkg,examples/bzlmod/runfiles,examples/bzlmod/tests,examples/bzlmod/tests/other_module,examples/bzlmod/whl_mods,examples/multi_python_versions/libs/my_lib,examples/multi_python_versions/requirements,examples/multi_python_versions/tests,examples/pip_install,examples/pip_parse,examples/pip_parse_vendored,examples/pip_repository_annotations,examples/py_proto_library,tests/compile_pip_requirements,tests/compile_pip_requirements_test_from_external_workspace,tests/ignore_root_user_error,tests/pip_repository_entry_points
 
 test --test_output=errors
 

examples/bzlmod/other_module/BUILD.bazel

@@ -0,0 +1,9 @@
+load("@python_versions//3.11:defs.bzl", compile_pip_requirements_311 = "compile_pip_requirements")
+
+# NOTE: To update the requirements, you need to uncomment the rules_python
+# override in the MODULE.bazel.
+compile_pip_requirements_311(
+    name = "requirements",
+    requirements_in = "requirements.in",
+    requirements_txt = "requirements_lock_3_11.txt",
+)

examples/bzlmod/other_module/MODULE.bazel

@@ -6,10 +6,20 @@ module(
 # that the parent module uses.
 bazel_dep(name = "rules_python", version = "")
 
-# It is not best practice to use a python.toolchian in
-# a submodule.  This code only exists to test that
-# we support doing this. This code is only for rules_python
-# testing purposes.
+# The story behind this commented out override:
+# This override is necessary to generate/update the requirements file
+# for this module. This is because running it via the outer
+# module doesn't work -- the `requirements.update` target can't find
+# the correct file to update.
+# Running in the submodule itself works, but submodules using overrides
+# is considered an error until Bazel 6.3, which prevents the outer module
+# from depending on this module.
+# So until 6.3 and higher is the minimum, we leave this commented out.
+# local_path_override(
+#     module_name = "rules_python",
+#     path = "../../..",
+# )
+
 PYTHON_NAME_39 = "python_3_9"
 
 PYTHON_NAME_311 = "python_3_11"
@@ -29,6 +39,20 @@ python.toolchain(
 # created by the above python.toolchain calls.
 use_repo(
     python,
+    "python_versions",
     PYTHON_NAME_39,
     PYTHON_NAME_311,
 )
+
+pip = use_extension("@rules_python//python/extensions:pip.bzl", "pip")
+pip.parse(
+    hub_name = "other_module_pip",
+    # NOTE: This version must be different than the root module's
+    # default python version.
+    # This is testing that a sub-module can use pip.parse() and only specify
+    # Python versions that DON'T include whatever the root-module's default
+    # Python version is.
+    python_version = "3.11",
+    requirements_lock = ":requirements_lock_3_11.txt",
+)
+use_repo(pip, "other_module_pip")

examples/bzlmod/other_module/other_module/pkg/BUILD.bazel

@@ -1,4 +1,7 @@
-load("@python_3_11//:defs.bzl", py_binary_311 = "py_binary")
+load(
+    "@python_3_11//:defs.bzl",
+    py_binary_311 = "py_binary",
+)
 load("@rules_python//python:defs.bzl", "py_library")
 
 py_library(
@@ -13,11 +16,16 @@ py_library(
 # used only when you need to support multiple versions of Python
 # in the same project.
 py_binary_311(
-    name = "lib_311",
-    srcs = ["lib.py"],
+    name = "bin",
+    srcs = ["bin.py"],
     data = ["data/data.txt"],
+    main = "bin.py",
     visibility = ["//visibility:public"],
-    deps = ["@rules_python//python/runfiles"],
+    deps = [
+        ":lib",
+        "@other_module_pip//absl_py",
+        "@rules_python//python/runfiles",
+    ],
 )
 
 exports_files(["data/data.txt"])

examples/bzlmod/other_module/other_module/pkg/bin.py

@@ -0,0 +1,6 @@
+import sys
+
+import absl
+
+print("Python version:", sys.version)
+print("Module 'absl':", absl)

examples/bzlmod/other_module/requirements.in

@@ -0,0 +1 @@
+absl-py

examples/bzlmod/other_module/requirements_lock_3_11.txt

@@ -0,0 +1,10 @@
+#
+# This file is autogenerated by pip-compile with Python 3.11
+# by the following command:
+#
+#    bazel run //other_module/pkg:requirements.update
+#
+absl-py==1.4.0 \
+    --hash=sha256:0d3fe606adfa4f7db64792dd4c7aee4ee0c38ab75dfd353b7a83ed3e957fcb47 \
+    --hash=sha256:d2c244d01048ba476e7c080bd2c6df5e141d211de80223460d5b3b8a2a58433d
+    # via -r other_module/pkg/requirements.in

examples/bzlmod/tests/other_module/BUILD.bazel

@@ -0,0 +1,14 @@
+# Tests to verify the root module can interact with the "other_module"
+# submodule.
+#
+# Note that other_module is seen as "our_other_module" due to repo-remapping
+# in the root module.
+
+load("@bazel_skylib//rules:build_test.bzl", "build_test")
+
+build_test(
+    name = "other_module_bin_build_test",
+    targets = [
+        "@our_other_module//other_module/pkg:bin",
+    ],
+)

python/extensions/pip.bzl

@@ -291,23 +291,18 @@ def _pip_impl(module_ctx):
 
     for hub_name, whl_map in hub_whl_map.items():
         for whl_name, version_map in whl_map.items():
-            if DEFAULT_PYTHON_VERSION not in version_map:
-                fail((
-                    "Default python version '{version}' missing in pip " +
-                    "hub '{hub}': update your pip.parse() calls so that " +
-                    'includes `python_version = "{version}"`'
-                ).format(
-                    version = DEFAULT_PYTHON_VERSION,
-                    hub = hub_name,
-                ))
+            if DEFAULT_PYTHON_VERSION in version_map:
+                whl_default_version = DEFAULT_PYTHON_VERSION
+            else:
+                whl_default_version = None
 
             # Create the alias repositories which contains different select
             # statements  These select statements point to the different pip
             # whls that are based on a specific version of Python.
             whl_library_alias(
                 name = hub_name + "_" + whl_name,
                 wheel_name = whl_name,
-                default_version = DEFAULT_PYTHON_VERSION,
+                default_version = whl_default_version,
                 version_map = version_map,
             )
 
@@ -363,7 +358,7 @@ is used, this attribute defaults to that version of Python.
             mandatory = False,
             doc = """\
 A dict of labels to wheel names that is typically generated by the whl_modifications.
-The labels are JSON config files describing the modifications.           
+The labels are JSON config files describing the modifications.
 """,
         ),
     }, **pip_repository_attrs)
@@ -396,7 +391,7 @@ executable.""",
         ),
         "copy_files": attr.string_dict(
             doc = """\
-(dict, optional): A mapping of `src` and `out` files for 
+(dict, optional): A mapping of `src` and `out` files for
 [@bazel_skylib//rules:copy_file.bzl][cf]""",
         ),
         "data": attr.string_list(
@@ -457,10 +452,10 @@ the BUILD files for wheels.
             attrs = _pip_parse_ext_attrs(),
             doc = """\
 This tag class is used to create a pip hub and all of the spokes that are part of that hub.
-This tag class reuses most of the pip attributes that are found in 
+This tag class reuses most of the pip attributes that are found in
 @rules_python//python/pip_install:pip_repository.bzl.
-The exceptions are it does not use the args 'repo_prefix', 
-and 'incompatible_generate_aliases'.  We set the repository prefix 
+The exceptions are it does not use the args 'repo_prefix',
+and 'incompatible_generate_aliases'.  We set the repository prefix
 for the user and the alias arg is always True in bzlmod.
 """,
         ),
@@ -484,7 +479,7 @@ def _whl_mods_repo_impl(rctx):
 
 _whl_mods_repo = repository_rule(
     doc = """\
-This rule creates json files based on the whl_mods attribute. 
+This rule creates json files based on the whl_mods attribute.
 """,
     implementation = _whl_mods_repo_impl,
     attrs = {

python/pip.bzl

@@ -22,6 +22,25 @@ load(":versions.bzl", "MINOR_MAPPING")
 compile_pip_requirements = _compile_pip_requirements
 package_annotation = _package_annotation
 
+_NO_MATCH_ERROR_MESSAGE_TEMPLATE = """\
+No matching wheel for current configuration's Python version.
+
+The current build configuration's Python version doesn't match any of the Python
+versions available for this wheel. This wheel supports the following Python versions:
+    {supported_versions}
+
+As matched by the `@{rules_python}//python/config_settings:is_python_<version>`
+configuration settings.
+
+To determine the current configuration's Python version, run:
+    `bazel config <config id>` (shown further below)
+and look for
+    {rules_python}//python/config_settings:python_version
+
+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.
+"""
+
 def pip_install(requirements = None, name = "pip", **kwargs):
     """Accepts a locked/compiled requirements file and installs the dependencies listed within.
 
@@ -260,13 +279,10 @@ _multi_pip_parse = repository_rule(
 
 def _whl_library_alias_impl(rctx):
     rules_python = rctx.attr._rules_python_workspace.workspace_name
-    if rctx.attr.default_version not in rctx.attr.version_map:
-        fail(
-            """
-Unable to find '{}' in your version map, you may need to update your requirement files.
-        """.format(rctx.attr.version_map),
-        )
-    default_repo_prefix = rctx.attr.version_map[rctx.attr.default_version]
+    if rctx.attr.default_version:
+        default_repo_prefix = rctx.attr.version_map[rctx.attr.default_version]
+    else:
+        default_repo_prefix = None
     version_map = rctx.attr.version_map.items()
     build_content = ["# Generated by python/pip.bzl"]
     for alias_name in ["pkg", "whl", "data", "dist_info"]:
@@ -289,6 +305,7 @@ def _whl_library_render_alias_target(
     # is canonical, so we have to add a second @.
     if BZLMOD_ENABLED:
         rules_python = "@" + rules_python
+
     alias = ["""\
 alias(
     name = "{alias_name}",
@@ -304,23 +321,42 @@ alias(
             ),
             rules_python = rules_python,
         ))
-    alias.append("""\
-        "//conditions:default": "{default_actual}",
-    }}),
-    visibility = ["//visibility:public"],
-)""".format(
+    if default_repo_prefix:
         default_actual = "@{repo_prefix}{wheel_name}//:{alias_name}".format(
             repo_prefix = default_repo_prefix,
             wheel_name = wheel_name,
             alias_name = alias_name,
-        ),
-    ))
+        )
+        alias.append('        "//conditions:default": "{default_actual}",'.format(
+            default_actual = default_actual,
+        ))
+
+    alias.append("    },")  # Close select expression condition dict
+    if not default_repo_prefix:
+        supported_versions = sorted([python_version for python_version, _ in version_map])
+        alias.append('    no_match_error="""{}""",'.format(
+            _NO_MATCH_ERROR_MESSAGE_TEMPLATE.format(
+                supported_versions = ", ".join(supported_versions),
+                rules_python = rules_python,
+            ),
+        ))
+    alias.append("    ),")  # Close the select expression
+    alias.append('    visibility = ["//visibility:public"],')
+    alias.append(")")  # Close the alias() expression
     return "\n".join(alias)
 
 whl_library_alias = repository_rule(
     _whl_library_alias_impl,
     attrs = {
-        "default_version": attr.string(mandatory = True),
+        "default_version": attr.string(
+            mandatory = False,
+            doc = "Optional Python version in major.minor format, e.g. '3.10'." +
+                  "The Python version of the wheel to use when the versions " +
+                  "from `version_map` don't match. This allows the default " +
+                  "(version unaware) rules to match and select a wheel. If " +
+                  "not specified, then the default rules won't be able to " +
+                  "resolve a wheel and an error will occur.",
+        ),
         "version_map": attr.string_dict(mandatory = True),
         "wheel_name": attr.string(mandatory = True),
         "_rules_python_workspace": attr.label(default = Label("//:WORKSPACE")),