From dd1b498bc060d9c519f7c903b7e8c3ae3bf5311d Mon Sep 17 00:00:00 2001
From: Adam Kocoloski <adam.kocoloski@airbnb.com>
Date: Wed, 9 Sep 2026 14:06:58 -0400
Subject: [PATCH 1/2] fix(bzlmod): convert coverage_tool Label to string

The `coverage_tool` attr on `python.single_version_platform_override`
is declared as `attr.label` (so bzlmod resolves the target relative to
the calling module), but the value was passed through unchanged to
`python_repository`, which declares `coverage_tool` as a string.
This caused a type mismatch error at module-extension evaluation time
whenever `coverage_tool` was set. Convert the Label to its canonical
string form before storing it.

Fixes #2570.
---
 news/2570.fixed.md            |  2 ++
 python/private/python.bzl     |  5 ++++-
 tests/python/python_tests.bzl | 15 +++++++++++++--
 3 files changed, 19 insertions(+), 3 deletions(-)
 create mode 100644 news/2570.fixed.md

diff --git a/news/2570.fixed.md b/news/2570.fixed.md
new file mode 100644
index 0000000000..194f00142b
--- /dev/null
+++ b/news/2570.fixed.md
@@ -0,0 +1,2 @@
+(bzlmod) Fixed a type mismatch error when using {obj}`coverage_tool` with
+{obj}`python.single_version_platform_override` ([#2570](https://github.com/bazel-contrib/rules_python/issues/2570)).
diff --git a/python/private/python.bzl b/python/private/python.bzl
index 70a3bd9770..9fed9393ae 100644
--- a/python/private/python.bzl
+++ b/python/private/python.bzl
@@ -626,7 +626,10 @@ def _process_single_version_platform_overrides(*, tag, _fail = fail, default):
         available_versions[tag.python_version] = {}
 
     if tag.coverage_tool:
-        available_versions[tag.python_version].setdefault("coverage_tool", {})[tag.platform] = tag.coverage_tool
+        # NOTE: tag.coverage_tool is a Label (so that it is resolved relative to the
+        # calling module), but downstream (python_repository.coverage_tool) it is
+        # consumed as a string, so convert it to its canonical string form here.
+        available_versions[tag.python_version].setdefault("coverage_tool", {})[tag.platform] = str(tag.coverage_tool)
     if tag.patch_strip:
         available_versions[tag.python_version].setdefault("patch_strip", {})[tag.platform] = tag.patch_strip
     if tag.patches:
diff --git a/tests/python/python_tests.bzl b/tests/python/python_tests.bzl
index 5636b7e32c..ef7ccf034f 100644
--- a/tests/python/python_tests.bzl
+++ b/tests/python/python_tests.bzl
@@ -481,7 +481,11 @@ def _test_add_new_version(env):
                 ],
                 single_version_platform_override = [
                     python_ext.single_version_platform_override(
-                        coverage_tool = "specific_cov_tool",
+                        # `coverage_tool` is declared as `attr.label` on the tag class
+                        # (so bzlmod resolves it relative to the calling module), so it
+                        # is a `Label`, not a plain `str`, by the time it reaches here.
+                        # See https://github.com/bazel-contrib/rules_python/issues/2570.
+                        coverage_tool = Label("@my_module//:specific_cov_tool"),
                         patch_strip = 2,
                         patches = ["specific-patch.txt"],
                         platform = "aarch64-unknown-linux-gnu",
@@ -509,8 +513,15 @@ def _test_add_new_version(env):
         "strip_prefix": {"aarch64-unknown-linux-gnu": "prefix"},
         "url": {"aarch64-unknown-linux-gnu": ["example.org"]},
     })
+
+    # The Label must be converted to its canonical string form: `python_repository`
+    # (which ultimately consumes this value) declares `coverage_tool` as `attr.string`.
+    coverage_tool = py.config.default["tool_versions"]["3.13.99"]["coverage_tool"]["aarch64-unknown-linux-gnu"]
+    env.expect.that_str(type(coverage_tool)).equals("string")
+    env.expect.that_str(coverage_tool).equals(str(Label("@my_module//:specific_cov_tool")))
+
     env.expect.that_dict(py.config.default["tool_versions"]["3.13.99"]).contains_exactly({
-        "coverage_tool": {"aarch64-unknown-linux-gnu": "specific_cov_tool"},
+        "coverage_tool": {"aarch64-unknown-linux-gnu": coverage_tool},
         "patch_strip": {"aarch64-unknown-linux-gnu": 2},
         "patches": {"aarch64-unknown-linux-gnu": ["specific-patch.txt"]},
         "sha256": {"aarch64-unknown-linux-gnu": "deadb00f"},

From 28cc9eb1a417f98ba4ca4c01e3dcef77a93c7b5a Mon Sep 17 00:00:00 2001
From: Adam Kocoloski <kocolosk@apache.org>
Date: Wed, 9 Sep 2026 16:21:16 -0400
Subject: [PATCH 2/2] Confirm to 80 column limit

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
---
 news/2570.fixed.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/news/2570.fixed.md b/news/2570.fixed.md
index 194f00142b..f31115d4f3 100644
--- a/news/2570.fixed.md
+++ b/news/2570.fixed.md
@@ -1,2 +1,3 @@
 (bzlmod) Fixed a type mismatch error when using {obj}`coverage_tool` with
-{obj}`python.single_version_platform_override` ([#2570](https://github.com/bazel-contrib/rules_python/issues/2570)).
+{obj}`python.single_version_platform_override`
+([#2570](https://github.com/bazel-contrib/rules_python/issues/2570)).