Allow Shiny Express syntax in Quarto Dashboards

Author: wch

shiny/express/__init__.py

@@ -37,6 +37,16 @@
 session: _Session
 
 
+allow_express_in_core = False
+"""
+Control whether Shiny Express imports are allowed in Shiny Core apps.
+
+Normally Shiny Express imports are not allowed in Shiny Core apps because the mixing of
+the two could lead to confusion. However, there are special cases where we do allow
+this, like when using Shiny Express syntax in a Quarto Dashboard.
+"""
+
+
 # Note that users should use `from shiny.express import input` instead of `from shiny
 # import express` and acces via `express.input`. The former provides a static value for
 # `input`, but the latter is dynamic -- every time `express.input` is accessed, it

shiny/express/_is_express.py

@@ -51,13 +51,17 @@ def is_express_app(app: str, app_dir: str | None) -> bool:
     except Exception:
         return False
 
-    return detector.found_shiny_express_import
+    return detector.is_express_app()
 
 
 class DetectShinyExpressVisitor(ast.NodeVisitor):
     def __init__(self):
         super().__init__()
         self.found_shiny_express_import = False
+        self.found_shiny_express_in_core = False
+
+    def is_express_app(self) -> bool:
+        return self.found_shiny_express_import and not self.found_shiny_express_in_core
 
     def visit_Import(self, node: ast.Import) -> None:
         if any(alias.name == "shiny.express" for alias in node.names):
@@ -71,6 +75,20 @@ def visit_ImportFrom(self, node: ast.ImportFrom) -> None:
         ):
             self.found_shiny_express_import = True
 
+    def visit_Assign(self, node: ast.Assign) -> None:
+        for target in node.targets:
+            # Look for the statement `shiny.express.allow_express_in_core = True` at the
+            # top level.
+            if (
+                isinstance(target, ast.Attribute)
+                and isinstance(target.value, ast.Attribute)
+                and target.value.attr == "express"
+                and isinstance(target.value.value, ast.Name)
+                and target.value.value.id == "shiny"
+                and target.attr == "allow_express_in_core"
+            ):
+                self.found_shiny_express_in_core = True
+
     # Visit top-level nodes.
     def visit_Module(self, node: ast.Module) -> None:
         super().generic_visit(node)

shiny/express/_run.py

@@ -144,8 +144,15 @@ def set_result(x: object):
         get_top_level_recall_context_manager().__exit__(None, None, None)
 
         # If we're running as an Express app but there's also a top-level item named app
-        # which is a shiny.App object, the user probably made a mistake.
-        if "app" in var_context and isinstance(var_context["app"], App):
+        # which is a shiny.App object, the user probably made a mistake. The exception
+        # is if allow_express_in_core is True, which happens with Quarto Dashboards.
+        from . import allow_express_in_core
+
+        if (
+            not allow_express_in_core
+            and "app" in var_context
+            and isinstance(var_context["app"], App)
+        ):
             raise RuntimeError(
                 "This looks like a Shiny Express app because it imports shiny.express, "
                 "but it also looks like a Shiny Core app because it has a variable named "

shiny/quarto.py

@@ -73,6 +73,9 @@ def convert_code_cells_to_app_py(json_file: str | Path, app_file: str | Path) ->
 
 from pathlib import Path
 from shiny import App, Inputs, Outputs, Session, ui
+import shiny.express
+
+shiny.express.allow_express_in_core = True
 
 {"".join(global_code_cell_texts)}