Fix render.download not working in Express

shiny/api-examples/download_button/app-express.py

@@ -0,0 +1,15 @@
+import asyncio
+import random
+from datetime import date
+
+from shiny.express import render
+
+
+@render.download(
+    filename=lambda: f"新型-{date.today().isoformat()}-{random.randint(100,999)}.csv"
+)
+async def downloadData():
+    await asyncio.sleep(0.25)
+    yield "one,two,three\n"
+    yield "新,1,2\n"
+    yield "型,4,5\n"

shiny/express/_mock_session.py

@@ -17,6 +17,9 @@ def __init__(self):
         self.input = Inputs({})
         self.output = Outputs(cast(Session, self), self.ns, {}, {})
 
+    def __bool__(self) -> bool:
+        return False
+
     # This is needed so that Outputs don't throw an error.
     def _is_hidden(self, name: str) -> bool:
         return False

shiny/render/_render.py

@@ -688,7 +688,7 @@ def url() -> str:
         # not being None is because in Express, when the UI is rendered, this function
         # `render.download()()`  called once before any sessions have been started.
         session = get_current_session()
-        if session is not None:
+        if session:
             session._downloads[self.output_id] = DownloadInfo(
                 filename=self.filename,
                 content_type=self.media_type,