Skip rendering None in all situations (#1171)

* skip rendering none

* add changelog

* conditional render none should not reset state for sibling components

* minor renaming + better changelog

* misc fixes

* raises exceptiongroup

* skipif

* handle egroup in starlette

* final nocov

Author: rmorshea

docs/source/about/changelog.rst

@@ -35,6 +35,23 @@ Unreleased
   the overall responsiveness of your app, particularly when handling larger renders
   that would otherwise block faster renders from being processed.
 
+**Changed**
+
+- :pull:`1171` - Previously ``None``, when present in an HTML element, would render as
+  the string ``"None"``. Now ``None`` will not render at all. This is consistent with
+  how ``None`` is handled when returned from components. It also makes it easier to
+  conditionally render elements. For example, previously you would have needed to use a
+  fragment to conditionally render an element by writing
+  ``something if condition else html._()``. Now you can simply write
+  ``something if condition else None``.
+
+**Deprecated**
+
+- :pull:`1171` - The ``Stop`` exception. Recent releases of ``anyio`` have made this
+  exception difficult to use since it now raises an ``ExceptionGroup``. This exception
+  was primarily used for internal testing purposes and so is now deprecated.
+
+
 v1.0.2
 ------
 

src/py/reactpy/pyproject.toml

@@ -25,6 +25,7 @@ classifiers = [
   "Programming Language :: Python :: Implementation :: PyPy",
 ]
 dependencies = [
+  "exceptiongroup >=1.0",
   "typing-extensions >=3.10",
   "mypy-extensions >=0.4.3",
   "anyio >=3",

src/py/reactpy/reactpy/__init__.py

@@ -16,7 +16,6 @@
     use_state,
 )
 from reactpy.core.layout import Layout
-from reactpy.core.serve import Stop
 from reactpy.core.vdom import vdom
 from reactpy.utils import Ref, html_to_vdom, vdom_to_html
 

src/py/reactpy/reactpy/backend/starlette.py

@@ -7,6 +7,7 @@
 from dataclasses import dataclass
 from typing import Any, Callable
 
+from exceptiongroup import BaseExceptionGroup
 from starlette.applications import Starlette
 from starlette.middleware.cors import CORSMiddleware
 from starlette.requests import Request
@@ -137,8 +138,6 @@ async def serve_index(request: Request) -> HTMLResponse:
 def _setup_single_view_dispatcher_route(
     options: Options, app: Starlette, component: RootComponentConstructor
 ) -> None:
-    @app.websocket_route(str(STREAM_PATH))
-    @app.websocket_route(f"{STREAM_PATH}/{{path:path}}")
     async def model_stream(socket: WebSocket) -> None:
         await socket.accept()
         send, recv = _make_send_recv_callbacks(socket)
@@ -162,8 +161,16 @@ async def model_stream(socket: WebSocket) -> None:
                 send,
                 recv,
             )
-        except WebSocketDisconnect as error:
-            logger.info(f"WebSocket disconnect: {error.code}")
+        except BaseExceptionGroup as egroup:
+            for e in egroup.exceptions:
+                if isinstance(e, WebSocketDisconnect):
+                    logger.info(f"WebSocket disconnect: {e.code}")
+                    break
+            else:  # nocov
+                raise
+
+    app.add_websocket_route(str(STREAM_PATH), model_stream)
+    app.add_websocket_route(f"{STREAM_PATH}/{{path:path}}", model_stream)
 
 
 def _make_send_recv_callbacks(

src/py/reactpy/reactpy/core/layout.py

@@ -11,7 +11,7 @@
     wait,
 )
 from collections import Counter
-from collections.abc import Iterator
+from collections.abc import Sequence
 from contextlib import AsyncExitStack
 from logging import getLogger
 from typing import (
@@ -27,6 +27,7 @@
 from weakref import ref as weakref
 
 from anyio import Semaphore
+from typing_extensions import TypeAlias
 
 from reactpy.config import (
     REACTPY_ASYNC_RENDERING,
@@ -37,8 +38,10 @@
 from reactpy.core.types import (
     ComponentType,
     EventHandlerDict,
+    Key,
     LayoutEventMessage,
     LayoutUpdateMessage,
+    VdomChild,
     VdomDict,
     VdomJson,
 )
@@ -189,9 +192,7 @@ async def _render_component(
             # wrap the model in a fragment (i.e. tagName="") to ensure components have
             # a separate node in the model state tree. This could be removed if this
             # components are given a node in the tree some other way
-            wrapper_model: VdomDict = {"tagName": ""}
-            if raw_model is not None:
-                wrapper_model["children"] = [raw_model]
+            wrapper_model: VdomDict = {"tagName": "", "children": [raw_model]}
             await self._render_model(exit_stack, old_state, new_state, wrapper_model)
         except Exception as error:
             logger.exception(f"Failed to render {component}")
@@ -329,11 +330,11 @@ async def _render_model_children(
             await self._unmount_model_states(list(old_state.children_by_key.values()))
             return None
 
-        child_type_key_tuples = list(_process_child_type_and_key(raw_children))
+        children_info = _get_children_info(raw_children)
 
-        new_keys = {item[2] for item in child_type_key_tuples}
-        if len(new_keys) != len(raw_children):
-            key_counter = Counter(item[2] for item in child_type_key_tuples)
+        new_keys = {k for _, _, k in children_info}
+        if len(new_keys) != len(children_info):
+            key_counter = Counter(item[2] for item in children_info)
             duplicate_keys = [key for key, count in key_counter.items() if count > 1]
             msg = f"Duplicate keys {duplicate_keys} at {new_state.patch_path or '/'!r}"
             raise ValueError(msg)
@@ -345,7 +346,7 @@ async def _render_model_children(
             )
 
         new_state.model.current["children"] = []
-        for index, (child, child_type, key) in enumerate(child_type_key_tuples):
+        for index, (child, child_type, key) in enumerate(children_info):
             old_child_state = old_state.children_by_key.get(key)
             if child_type is _DICT_TYPE:
                 old_child_state = old_state.children_by_key.get(key)
@@ -420,17 +421,17 @@ async def _render_model_children_without_old_state(
         new_state: _ModelState,
         raw_children: list[Any],
     ) -> None:
-        child_type_key_tuples = list(_process_child_type_and_key(raw_children))
+        children_info = _get_children_info(raw_children)
 
-        new_keys = {item[2] for item in child_type_key_tuples}
-        if len(new_keys) != len(raw_children):
-            key_counter = Counter(item[2] for item in child_type_key_tuples)
+        new_keys = {k for _, _, k in children_info}
+        if len(new_keys) != len(children_info):
+            key_counter = Counter(k for _, _, k in children_info)
             duplicate_keys = [key for key, count in key_counter.items() if count > 1]
             msg = f"Duplicate keys {duplicate_keys} at {new_state.patch_path or '/'!r}"
             raise ValueError(msg)
 
         new_state.model.current["children"] = []
-        for index, (child, child_type, key) in enumerate(child_type_key_tuples):
+        for index, (child, child_type, key) in enumerate(children_info):
             if child_type is _DICT_TYPE:
                 child_state = _make_element_model_state(new_state, index, key)
                 await self._render_model(exit_stack, None, child_state, child)
@@ -609,7 +610,7 @@ def __init__(
         key: Any,
         model: Ref[VdomJson],
         patch_path: str,
-        children_by_key: dict[str, _ModelState],
+        children_by_key: dict[Key, _ModelState],
         targets_by_event: dict[str, str],
         life_cycle_state: _LifeCycleState | None = None,
     ):
@@ -720,16 +721,17 @@ async def get(self) -> _Type:
         return value
 
 
-def _process_child_type_and_key(
-    children: list[Any],
-) -> Iterator[tuple[Any, _ElementType, Any]]:
+def _get_children_info(children: list[VdomChild]) -> Sequence[_ChildInfo]:
+    infos: list[_ChildInfo] = []
     for index, child in enumerate(children):
-        if isinstance(child, dict):
+        if child is None:
+            continue
+        elif isinstance(child, dict):
             child_type = _DICT_TYPE
             key = child.get("key")
         elif isinstance(child, ComponentType):
             child_type = _COMPONENT_TYPE
-            key = getattr(child, "key", None)
+            key = child.key
         else:
             child = f"{child}"
             child_type = _STRING_TYPE
@@ -738,8 +740,12 @@ def _process_child_type_and_key(
         if key is None:
             key = index
 
-        yield (child, child_type, key)
+        infos.append((child, child_type, key))
 
+    return infos
+
+
+_ChildInfo: TypeAlias = tuple[Any, "_ElementType", Key]
 
 # used in _process_child_type_and_key
 _ElementType = NewType("_ElementType", int)

src/py/reactpy/reactpy/core/serve.py

@@ -3,6 +3,7 @@
 from collections.abc import Awaitable
 from logging import getLogger
 from typing import Callable
+from warnings import warn
 
 from anyio import create_task_group
 from anyio.abc import TaskGroup
@@ -24,7 +25,9 @@
 
 
 class Stop(BaseException):
-    """Stop serving changes and events
+    """Deprecated
+
+    Stop serving changes and events
 
     Raising this error will tell dispatchers to gracefully exit. Typically this is
     called by code running inside a layout to tell it to stop rendering.
@@ -42,7 +45,12 @@ async def serve_layout(
             async with create_task_group() as task_group:
                 task_group.start_soon(_single_outgoing_loop, layout, send)
                 task_group.start_soon(_single_incoming_loop, task_group, layout, recv)
-        except Stop:
+        except Stop:  # nocov
+            warn(
+                "The Stop exception is deprecated and will be removed in a future version",
+                UserWarning,
+                stacklevel=1,
+            )
             logger.info(f"Stopped serving {layout}")
 
 

src/py/reactpy/reactpy/core/types.py

@@ -91,7 +91,7 @@ async def __aexit__(
 VdomAttributes = Mapping[str, Any]
 """Describes the attributes of a :class:`VdomDict`"""
 
-VdomChild: TypeAlias = "ComponentType | VdomDict | str"
+VdomChild: TypeAlias = "ComponentType | VdomDict | str | None | Any"
 """A single child element of a :class:`VdomDict`"""
 
 VdomChildren: TypeAlias = "Sequence[VdomChild] | VdomChild"
@@ -100,14 +100,7 @@ async def __aexit__(
 
 class _VdomDictOptional(TypedDict, total=False):
     key: Key | None
-    children: Sequence[
-        # recursive types are not allowed yet:
-        # https://github.com/python/mypy/issues/731
-        ComponentType
-        | dict[str, Any]
-        | str
-        | Any
-    ]
+    children: Sequence[ComponentType | VdomChild]
     attributes: VdomAttributes
     eventHandlers: EventHandlerDict
     importSource: ImportSourceDict

src/py/reactpy/tests/test_core/test_layout.py

@@ -102,15 +102,6 @@ def SimpleComponent():
         )
 
 
-async def test_component_can_return_none():
-    @reactpy.component
-    def SomeComponent():
-        return None
-
-    async with reactpy.Layout(SomeComponent()) as layout:
-        assert (await layout.render())["model"] == {"tagName": ""}
-
-
 async def test_nested_component_layout():
     parent_set_state = reactpy.Ref(None)
     child_set_state = reactpy.Ref(None)
@@ -1310,3 +1301,45 @@ def child_2():
 
         assert child_1_render_count.current == 1
         assert child_2_render_count.current == 1
+
+
+async def test_none_does_not_render():
+    @component
+    def Root():
+        return html.div(None, Child())
+
+    @component
+    def Child():
+        return None
+
+    async with layout_runner(Layout(Root())) as runner:
+        tree = await runner.render()
+        assert tree == {
+            "tagName": "",
+            "children": [
+                {"tagName": "div", "children": [{"tagName": "", "children": []}]}
+            ],
+        }
+
+
+async def test_conditionally_render_none_does_not_trigger_state_change_in_siblings():
+    toggle_condition = Ref()
+    effect_run_count = Ref(0)
+
+    @component
+    def Root():
+        condition, toggle_condition.current = use_toggle(True)
+        return html.div("text" if condition else None, Child())
+
+    @component
+    def Child():
+        @reactpy.use_effect
+        def effect():
+            effect_run_count.current += 1
+
+    async with layout_runner(Layout(Root())) as runner:
+        await runner.render()
+        poll(lambda: effect_run_count.current).until_equals(1)
+        toggle_condition.current()
+        await runner.render()
+    assert effect_run_count.current == 1

src/py/reactpy/tests/test_core/test_serve.py

@@ -1,7 +1,9 @@
 import asyncio
+import sys
 from collections.abc import Sequence
 from typing import Any
 
+import pytest
 from jsonpointer import set_pointer
 
 import reactpy
@@ -31,7 +33,7 @@ async def send(patch):
         changes.append(patch)
         sem.release()
         if not events_to_inject:
-            raise reactpy.Stop()
+            raise Exception("Stop running")
 
     async def recv():
         await sem.acquire()
@@ -90,10 +92,12 @@ def Counter():
     return reactpy.html.div({EVENT_NAME: handler, "count": count})
 
 
+@pytest.mark.skipif(sys.version_info < (3, 11), reason="ExceptionGroup not available")
 async def test_dispatch():
     events, expected_model = make_events_and_expected_model()
     changes, send, recv = make_send_recv_callbacks(events)
-    await asyncio.wait_for(serve_layout(Layout(Counter()), send, recv), 1)
+    with pytest.raises(ExceptionGroup):
+        await asyncio.wait_for(serve_layout(Layout(Counter()), send, recv), 1)
     assert_changes_produce_expected_model(changes, expected_model)