fix(rules): make the srcs trully optional

With this PR we mark the srcs attribute as optional as we can
leverage the `main_module` to just run things from the deps.

Fixes bazel-contrib#2765

Author: aignas

CHANGELOG.md

@@ -76,6 +76,8 @@ Unreleased changes template.
 * (pypi) The PyPI extension will no longer write the lock file entries as the
   extension has been marked reproducible.
   Fixes [#2434](https://github.com/bazel-contrib/rules_python/issues/2434).
+* (rules) {attr}`py_binary.srcs` and {attr}`py_test.srcs` is no longer mandatory when
+  `main_module` is specified.
 
 [20250317]: https://github.com/astral-sh/python-build-standalone/releases/tag/20250317
 

python/private/py_executable.bzl

@@ -786,7 +786,7 @@ def _create_stage1_bootstrap(
         )
         template = runtime.bootstrap_template
         subs["%shebang%"] = runtime.stub_shebang
-    else:
+    elif main_py:
         if (ctx.configuration.coverage_enabled and
             runtime and
             runtime.coverage_tool):
@@ -807,6 +807,8 @@ def _create_stage1_bootstrap(
         subs["%import_all%"] = ("True" if ctx.fragments.bazel_py.python_import_all_repositories else "False")
         subs["%imports%"] = ":".join(imports.to_list())
         subs["%main%"] = "{}/{}".format(ctx.workspace_name, main_py.short_path)
+    else:
+        fail("'main' or 'srcs' must be specified")
 
     ctx.actions.expand_template(
         template = template,
@@ -1888,7 +1890,6 @@ def create_executable_rule_builder(implementation, **kwargs):
         ),
         **kwargs
     )
-    builder.attrs.get("srcs").set_mandatory(True)
     return builder
 
 def cc_configure_features(

tests/base_rules/py_executable_base_tests.bzl

@@ -24,7 +24,7 @@ load("//python/private:util.bzl", "IS_BAZEL_7_OR_HIGHER")  # buildifier: disable
 load("//tests/base_rules:base_tests.bzl", "create_base_tests")
 load("//tests/base_rules:util.bzl", "WINDOWS_ATTR", pt_util = "util")
 load("//tests/support:py_executable_info_subject.bzl", "PyExecutableInfoSubject")
-load("//tests/support:support.bzl", "CC_TOOLCHAIN", "CROSSTOOL_TOP", "LINUX_X86_64", "WINDOWS_X86_64")
+load("//tests/support:support.bzl", "BOOTSTRAP_IMPL", "CC_TOOLCHAIN", "CROSSTOOL_TOP", "LINUX_X86_64", "WINDOWS_X86_64")
 
 _tests = []
 
@@ -365,6 +365,50 @@ def _test_py_runtime_info_provided_impl(env, target):
 
 _tests.append(_test_py_runtime_info_provided)
 
+def _test_no_srcs_script_bootstrap(name, config):
+    rt_util.helper_target(
+        config.rule,
+        name = name + "_subject",
+        main_module = "dummy",
+    )
+    analysis_test(
+        name = name,
+        impl = _test_no_srcs_script_bootstrap_impl,
+        target = name + "_subject",
+        config_settings = {
+            BOOTSTRAP_IMPL: "script",
+            "//command_line_option:platforms": [LINUX_X86_64],
+        },
+    )
+
+def _test_no_srcs_script_bootstrap_impl(env, target):
+    env.expect.that_target(target).default_outputs().contains(
+        "{package}/{test_name}_subject",
+    )
+
+_tests.append(_test_no_srcs_script_bootstrap)
+
+def _test_no_srcs(name, config):
+    rt_util.helper_target(
+        config.rule,
+        name = name + "_subject",
+        main_module = "dummy",
+    )
+    analysis_test(
+        name = name,
+        impl = _test_no_srcs_impl,
+        target = name + "_subject",
+        config_settings = {
+            "//command_line_option:platforms": [LINUX_X86_64],
+        },
+        expect_failure = True,
+    )
+
+def _test_no_srcs_impl(_, __):
+    pass
+
+_tests.append(_test_no_srcs)
+
 # Can't test this -- mandatory validation happens before analysis test
 # can intercept it
 # TODO(#1069): Once re-implemented in Starlark, modify rule logic to make this

tests/support/support.bzl

@@ -35,6 +35,7 @@ CROSSTOOL_TOP = Label("//tests/support/cc_toolchains:cc_toolchain_suite")
 # str() around Label() is necessary because rules_testing's config_settings
 # doesn't accept yet Label objects.
 ADD_SRCS_TO_RUNFILES = str(Label("//python/config_settings:add_srcs_to_runfiles"))
+BOOTSTRAP_IMPL = str(Label("//python/config_settings:bootstrap_impl"))
 EXEC_TOOLS_TOOLCHAIN = str(Label("//python/config_settings:exec_tools_toolchain"))
 PRECOMPILE = str(Label("//python/config_settings:precompile"))
 PRECOMPILE_SOURCE_RETENTION = str(Label("//python/config_settings:precompile_source_retention"))