Inline `functools.Placeholder` lookup

Partial normalization assigned the optional Python 3.14 placeholder
sentinel separately from its only conditional consumer.

Bind the sentinel with a walrus expression directly in the guard while
retaining the `getattr()` fallback for older Python versions.

Review: PR #481 (goodboy)
https://github.com/goodboy/tractor/pull/481#pullrequestreview-5012942328

Prompt-IO: ai/prompt-io/opencode/20260825T021319Z_ce430fca_prompt_io.md

(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))

ai/prompt-io/opencode/20260825T021319Z_ce430fca_prompt_io.md

@@ -0,0 +1,32 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+session: d9d7df2c-7044-463f-8768-ec024718eac9
+timestamp: 2026-08-25T02:13:19Z
+git_ref: ce430fca
+scope: code
+substantive: true
+raw_file: 20260825T021319Z_ce430fca_prompt_io.raw.md
+---
+
+## Prompt
+
+Continue PR #481 review remediation after committing provisional child
+registration clarifications. The human selected "keep" for inlining the
+guarded `functools.Placeholder` lookup with a walrus assignment.
+
+## Response summary
+
+Remove the standalone placeholder assignment and bind the optional
+Python 3.14 sentinel directly in the existing conditional while
+preserving compatibility behavior.
+
+## Files changed
+
+- `tractor/to_actor/_api.py` - inline placeholder feature detection.
+
+## Human edits
+
+The human explicitly selected "keep" after receiving keep/defer/drop
+options for this isolated cleanup. No direct source-line edits were made
+by the human.

ai/prompt-io/opencode/20260825T021319Z_ce430fca_prompt_io.raw.md

@@ -0,0 +1,19 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+timestamp: 2026-08-25T02:13:19Z
+git_ref: ce430fca
+diff_cmd: git diff HEAD~1..HEAD
+---
+
+Implement the approved PR #481 review cleanup for Python 3.14 partial
+placeholder detection.
+
+> `git diff HEAD~1..HEAD -- tractor/to_actor/_api.py`
+
+Inline the guarded `functools.Placeholder` lookup into the existing
+condition with a walrus assignment, preserving fallback behavior when
+the attribute is unavailable.
+
+Run partial/placeholder normalization tests and the full `to_actor`
+suite.

tractor/to_actor/_api.py

@@ -116,13 +116,14 @@ def _normalize_call(
 
         # `functools.Placeholder` was added in Python 3.14. Drop
         # this `getattr()` guard once 3.14 is the minimum version.
-        placeholder = getattr(
+        if (
+            (
+                placeholder := getattr(
                     functools,
                     'Placeholder',
                     None,
                 )
-        if (
-            placeholder is not None
+            ) is not None
             and
             any(
                 arg is placeholder