Don't run effects created in a MockSession

shiny/express/__init__.py

@@ -2,8 +2,12 @@
 
 # Import these with underscore names so they won't show in autocomplete from the Python
 # console.
-from ..session import Inputs as _Inputs, Outputs as _Outputs, Session as _Session
-from ..session import _utils as _session_utils
+from ..session import (
+    Inputs as _Inputs,
+    Outputs as _Outputs,
+    Session as _Session,
+    get_current_session as _get_current_session,
+)
 from .. import render
 from . import ui
 from ._is_express import is_express_app
@@ -39,42 +43,10 @@
 # cases, but when it fails, it will be very confusing.
 def __getattr__(name: str) -> object:
     if name == "input":
-        return _get_current_session_or_mock().input
+        return _get_current_session().input  # pyright: ignore
     elif name == "output":
-        return _get_current_session_or_mock().output
+        return _get_current_session().output  # pyright: ignore
     elif name == "session":
-        return _get_current_session_or_mock()
+        return _get_current_session()
 
     raise AttributeError(f"Module 'shiny.express' has no attribute '{name}'")
-
-
-# A very bare-bones mock session class that is used only in shiny.express.
-class _MockSession:
-    def __init__(self):
-        from typing import cast
-
-        from .._namespaces import Root
-
-        self.input = _Inputs({})
-        self.output = _Outputs(cast(_Session, self), Root, {}, {})
-
-    # This is needed so that Outputs don't throw an error.
-    def _is_hidden(self, name: str) -> bool:
-        return False
-
-
-_current_mock_session: _MockSession | None = None
-
-
-def _get_current_session_or_mock() -> _Session:
-    from typing import cast
-
-    session = _session_utils.get_current_session()
-    if session is None:
-        global _current_mock_session
-        if _current_mock_session is None:
-            _current_mock_session = _MockSession()
-        return cast(_Session, _current_mock_session)
-
-    else:
-        return session

shiny/express/_mock_session.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+import textwrap
+from typing import Awaitable, Callable, cast
+
+from .._namespaces import Root
+from ..session import Inputs, Outputs, Session
+
+all = ("MockSession",)
+
+
+# A very bare-bones mock session class that is used only in shiny.express's UI rendering
+# phase.
+class MockSession:
+    def __init__(self):
+        self.ns = Root
+        self.input = Inputs({})
+        self.output = Outputs(cast(Session, self), self.ns, {}, {})
+
+    # This is needed so that Outputs don't throw an error.
+    def _is_hidden(self, name: str) -> bool:
+        return False
+
+    def on_ended(
+        self,
+        fn: Callable[[], None] | Callable[[], Awaitable[None]],
+    ) -> Callable[[], None]:
+        return lambda: None
+
+    def __getattr__(self, name: str):
+        raise AttributeError(
+            textwrap.dedent(
+                f"""
+            The session attribute `{name}` is not yet available for use. Since this code
+            will run again when the session is initialized, you can use `if session:` to
+            only run this code when the session is established.
+        """
+            )
+        )

shiny/express/_run.py

@@ -8,7 +8,8 @@
 from htmltools import Tag, TagList
 
 from .._app import App
-from ..session import Inputs, Outputs, Session
+from ..session import Inputs, Outputs, Session, session_context
+from ._mock_session import MockSession
 from ._recall_context import RecallContextManager
 from .expressify_decorator._func_displayhook import _expressify_decorator_function_def
 from .expressify_decorator._node_transformers import (
@@ -33,13 +34,15 @@ def wrap_express_app(file: Path) -> App:
         A `shiny.App` object.
     """
     try:
-        # We tagify here, instead of waiting for the App object to do it when it wraps
-        # the UI in a HTMLDocument and calls render() on it. This is because
-        # AttributeErrors can be thrown during the tagification process, and we need to
-        # catch them here and convert them to a different type of error, because uvicorn
-        # specifically catches AttributeErrors and prints an error message that is
-        # misleading for Shiny Express. https://github.com/posit-dev/py-shiny/issues/937
-        app_ui = run_express(file).tagify()
+        with session_context(cast(Session, MockSession())):
+            # We tagify here, instead of waiting for the App object to do it when it wraps
+            # the UI in a HTMLDocument and calls render() on it. This is because
+            # AttributeErrors can be thrown during the tagification process, and we need to
+            # catch them here and convert them to a different type of error, because uvicorn
+            # specifically catches AttributeErrors and prints an error message that is
+            # misleading for Shiny Express. https://github.com/posit-dev/py-shiny/issues/937
+            app_ui = run_express(file).tagify()
+
     except AttributeError as e:
         raise RuntimeError(e) from e
 

shiny/reactive/_reactives.py

@@ -480,6 +480,7 @@ def __init__(
         self.__name__ = fn.__name__
         self.__doc__ = fn.__doc__
 
+        from ..express._mock_session import MockSession
         from ..render.renderer import Renderer
 
         if isinstance(fn, Renderer):
@@ -514,6 +515,12 @@ def __init__(
             # If no session is provided, autodetect the current session (this
             # could be None if outside of a session).
             session = get_current_session()
+
+        if isinstance(session, MockSession):
+            # If we're in a MockSession, then don't actually set up this Effect -- we
+            # don't want it to try to run later.
+            return
+
         self._session = session
 
         if self._session is not None: