From 2c331f67ea514fc510e40e3a99cbaa6a8c98eb9b Mon Sep 17 00:00:00 2001
From: gable-github <gabryelsoh@gmail.com>
Date: Fri, 16 May 2025 15:07:40 -0700
Subject: [PATCH] fix: raise TypeError when @app.template_filter is used
 without parentheses

---
 src/flask/sansio/app.py  | 9 +++++++++
 tests/test_templating.py | 8 ++++++++
 2 files changed, 17 insertions(+)

diff --git a/src/flask/sansio/app.py b/src/flask/sansio/app.py
index ceab45cb5e..05c75b5dd6 100644
--- a/src/flask/sansio/app.py
+++ b/src/flask/sansio/app.py
@@ -672,10 +672,19 @@ def template_filter(
           def reverse(s):
               return s[::-1]
 
+        You must include parentheses when using this decorator,
+        even if no arguments are provided.
+
         :param name: the optional name of the filter, otherwise the
                      function name will be used.
         """
 
+        if callable(name):
+            raise TypeError(
+                "Did you mean to use @app.template_filter()? "
+                "Missing parentheses () in decorator"
+            )
+
         def decorator(f: T_template_filter) -> T_template_filter:
             self.add_template_filter(f, name=name)
             return f
diff --git a/tests/test_templating.py b/tests/test_templating.py
index c9fb3754a3..423b0134e6 100644
--- a/tests/test_templating.py
+++ b/tests/test_templating.py
@@ -130,6 +130,14 @@ def my_reverse(s):
     assert app.jinja_env.filters["my_reverse"]("abcd") == "dcba"
 
 
+def test_template_filter_requires_parentheses(app):
+    with pytest.raises(TypeError):
+
+        @app.template_filter
+        def my_reverse(s):
+            return s[::-1]
+
+
 def test_add_template_filter(app):
     def my_reverse(s):
         return s[::-1]