Close #163. Don't run widget code if the shiny session is a stub session

Author: cpsievert

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [UNRELEASED]
 
 * Fixed an issue where widgets would sometimes fail to render in a Quarto document. (#159)
+* Fixed an issue where importing shinywidgets before a ipywidget implementation can sometimes error in a Shiny Express app. (#163)
 
 ## [0.3.3] - 2024-08-13
 

shinywidgets/_shinywidgets.py

@@ -45,6 +45,10 @@ def init_shiny_widget(w: Widget):
         raise RuntimeError(
             "shinywidgets requires that all ipywidgets be constructed within an active Shiny session"
         )
+    # Wait until we're in a "real" session before doing anything
+    # (i.e., on the 1st run of an Express app, it's too early to do anything)
+    if session.is_stub_session():
+        return
     # Break out of any module-specific session. Otherwise, input.shinywidgets_comm_send
     # will be some module-specific copy.
     while hasattr(session, "_parent"):