From 0589d6cc8fb8beb050042a0b3f848711952d2632 Mon Sep 17 00:00:00 2001
From: Peter Lobsinger <peter@tecton.ai>
Date: Fri, 20 Jun 2025 17:34:00 -0400
Subject: [PATCH] Support providing py_test's main from a separate package

A call to `py_test` using a `main` from a separate package, such as

```
py_test(
    name = "repro",
    srcs = [
        "//common:test_main.py",  # Exported by //common:BUILD.bazel.
        "specific_test.py",       # A second srcs file is needed to force the failure.
    ],
    main = "//common:test_main.py",
)
```

works when using `py_test` from `rules_python` v1.4.1, but fails when
using `aspect_rules_py` v1.6.0:

```
$ bazel test test:repro
INFO: Invocation ID: e1a579a3-5f06-48a4-93ac-493157878ad1
ERROR: /Users/peter/repro/test/BUILD.bazel:4:8: in determine_main rule //test:repro.find_main:
Traceback (most recent call last):
        File "/private/var/tmp/_bazel_peter/76f5e50945d4cebf81e863f9c06c01bc/external/aspect_rules_py+/py/private/py_executable.bzl", line 73, column 55, in _determine_main_impl
                return DefaultInfo(files = depset([_determine_main(ctx)]))
        File "/private/var/tmp/_bazel_peter/76f5e50945d4cebf81e863f9c06c01bc/external/aspect_rules_py+/py/private/py_executable.bzl", line 46, column 17, in _determine_main
                fail("could not find '{}' as specified by 'main' attribute".format(proposed_main))
Error in fail: could not find '//common:test_main.py' as specified by 'main' attribute
ERROR: /Users/peter/repro/test/BUILD.bazel:4:8: Analysis of target '//test:repro.find_main' failed
ERROR: Analysis of target '//test:repro' failed; build aborted: Analysis failed
INFO: Elapsed time: 0.140s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully
ERROR: No test targets were found, yet testing was requested
```

FWIW, I'm not sure this usage pattern is desirable; maybe I'll be able
to do away with it in my codebase in the future. But it is an
incompatibility that makes `aspect_rules_py` not a drop-in replacement
for `rules_python`; I hit it while attempting to migrate.
---
 py/private/py_executable.bzl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/py/private/py_executable.bzl b/py/private/py_executable.bzl
index f9e5370a..6b6d7a4f 100644
--- a/py/private/py_executable.bzl
+++ b/py/private/py_executable.bzl
@@ -26,7 +26,7 @@ def _determine_main(ctx):
     """
     if ctx.attr.main:
         # Deviation from rules_python: allow a leading colon, e.g. `main = ":my_target"`
-        proposed_main = ctx.attr.main.removeprefix(":")
+        _, _, proposed_main = ctx.attr.main.rpartition(":")
         if not proposed_main.endswith(".py"):
             fail("main {} must end in '.py'".format(proposed_main))
     else: