Provide useful message in Express when input was not imported (#994)

Author: wch

shiny/express/_run.py

@@ -84,6 +84,7 @@ def set_result(x: object):
         var_context: dict[str, object] = {
             "__file__": file_path,
             display_decorator_func_name: _display_decorator_function_def,
+            "input": InputNotImportedShim(),
         }
 
         # Execute each top-level node in the AST
@@ -144,3 +145,16 @@ def reset_top_level_recall_context_manager() -> None:
 
 def get_top_level_recall_context_manager() -> RecallContextManager[Tag]:
     return _top_level_recall_context_manager
+
+
+class InputNotImportedShim:
+    # This is a dummy class that is used to provide a helpful error message when the
+    # user tries to access `input.x` but forgot to import `input`. If they do that, then
+    # it would get the builtin `input` function, and print an unhelpful error message:
+    #   RuntimeError: 'builtin_function_or_method' object has no attribute 'x'
+    # This class provides an error message that is more helpful.
+    def __getattr__(self, name: str):
+        raise AttributeError(
+            "Tried to access `input`, but it was not imported. "
+            "Perhaps you need `from shiny.express import input`?"
+        )