Enforce top-level imports via ruff PLC0415

CHANGES/1349.contrib.rst

@@ -0,0 +1,8 @@
+Enabled the ``PLC0415`` (import-outside-top-level) rule in
+``[tool.ruff.lint]``. Function-scoped imports must now opt in
+with ``# noqa: PLC0415`` plus a comment explaining the reason
+(typically avoiding a heavy optional dependency at import time).
+This catches a pattern that LLM contributors frequently introduce.
+There are currently no opt-outs in the tree, so the rule is
+enforced everywhere ``ruff check`` runs
+-- by :user:`bdraco`.

pyproject.toml

@@ -14,6 +14,12 @@ select = [
   # (ruff format) and the import sorter (ruff check --select I) come
   # from the same tool and rev.
   "I",
+  # Imports must live at the top of the module. Late/inside-function
+  # imports must be opted into explicitly with `# noqa: PLC0415` and a
+  # comment explaining why (e.g. avoiding a heavy optional dependency
+  # in the import path). Enforced because AI-assisted contributions
+  # frequently introduce function-scoped imports that are unnecessary.
+  "PLC0415",
   # pyupgrade: enforce modern Python syntax for the current
   # ``target-version`` (currently py310).
   "UP",