Merge pull request #4348 from Yourun-proger/fix_msg

Fix error message for `after_this_request` when used outside request context

CHANGES.rst

@@ -15,6 +15,8 @@ Unreleased
 -   ``app.json_encoder`` and ``json_decoder`` are only passed to
     ``dumps`` and ``loads`` if they have custom behavior. This improves
     performance, mainly on PyPy. :issue:`4349`
+-   Clearer error message when ``after_this_request`` is used outside a
+    request context. :issue:`4333`
 
 
 Version 2.0.2

src/flask/ctx.py

@@ -130,7 +130,15 @@ def after_this_request(f: AfterRequestCallable) -> AfterRequestCallable:
 
     .. versionadded:: 0.9
     """
-    _request_ctx_stack.top._after_request_functions.append(f)
+    top = _request_ctx_stack.top
+
+    if top is None:
+        raise RuntimeError(
+            "This decorator can only be used when a request context is"
+            " active, such as within a view function."
+        )
+
+    top._after_request_functions.append(f)
     return f
 
 
@@ -159,12 +167,13 @@ def copy_current_request_context(f: t.Callable) -> t.Callable:
     .. versionadded:: 0.10
     """
     top = _request_ctx_stack.top
+
     if top is None:
         raise RuntimeError(
-            "This decorator can only be used at local scopes "
+            "This decorator can only be used when a request context is"
-            "when a request context is on the stack.  For instance within "
+            " active, such as within a view function."
-            "view functions."
         )
+
     reqctx = top.copy()
 
     def wrapper(*args, **kwargs):