Add docstrings to internal utility functions

Add docstrings to contextmanager_in_threadpool and
generate_encoders_by_class_tuples to document their purpose and
behavior for contributors navigating the codebase.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

fastapi/concurrency.py

@@ -18,12 +18,15 @@ _T = TypeVar("_T")
 async def contextmanager_in_threadpool(
     cm: AbstractContextManager[_T],
 ) -> AsyncGenerator[_T, None]:
-    # blocking __exit__ from running waiting on a free thread
+    """Run a synchronous context manager in a thread pool.
-    # can create race conditions/deadlocks if the context manager itself
+
-    # has its own internal pool (e.g. a database connection pool)
+    Enters and exits the context manager in a worker thread so that
-    # to avoid this we let __exit__ run without a capacity limit
+    blocking ``__enter__`` / ``__exit__`` calls do not block the async
-    # since we're creating a new limiter for each call, any non-zero limit
+    event loop.  The ``__exit__`` call uses its own
-    # works (1 is arbitrary)
+    :class:`~anyio.CapacityLimiter` to avoid deadlocks when the context
+    manager holds resources from a bounded pool (e.g. database
+    connections).
+    """
     exit_limiter = CapacityLimiter(1)
     try:
         yield await run_in_threadpool(cm.__enter__)

fastapi/encoders.py

@@ -98,6 +98,12 @@ ENCODERS_BY_TYPE: dict[type[Any], Callable[[Any], Any]] = {
 def generate_encoders_by_class_tuples(
     type_encoder_map: dict[Any, Callable[[Any], Any]],
 ) -> dict[Callable[[Any], Any], tuple[Any, ...]]:
+    """Invert a ``{type: encoder}`` mapping to ``{encoder: (types...)}``.
+
+    This allows :func:`jsonable_encoder` to perform efficient
+    ``isinstance`` checks against all types that share the same encoder
+    function in a single call.
+    """
     encoders_by_class_tuples: dict[Callable[[Any], Any], tuple[Any, ...]] = defaultdict(
         tuple
     )