Fix template_filter decorator to support both usage styles and update docstring; update CHANGES.rst

CHANGES.rst

@@ -6,6 +6,8 @@ Unreleased
 -   Drop support for Python 3.9. :pr:`5730`
 -   Remove previously deprecated code: ``__version__``. :pr:`5648`
 
+-   Fix template_filter decorator to correctly register filters and support usage with or without parentheses.
+-   Improve docstring for `template_filter` decorator for better clarity and examples.
 
 Version 3.1.1
 -------------

src/flask/sansio/app.py

@@ -664,37 +664,30 @@ class App(Scaffold):
     def template_filter(
         self, name: str | None = None
     ) -> t.Callable[[T_template_filter], T_template_filter]:
-        """A decorator that is used to register custom template filter.
+        """A decorator that is used to register custom template filters.
-        You can specify a name for the filter, otherwise the function
+              You can use this with or without parentheses. Example::
-        name will be used. Example::
 
-          @app.template_filter()
+              @app.template_filter
-          def reverse(s):
+              def double(x):
-              return s[::-1]
+                  return x * 2
 
-        :param name: the optional name of the filter, otherwise the
+              @app.template_filter()
-                     function name will be used.
+              def reverse(s):
+                  return s[::-1]
 
+            :param name: the optional name of the filter, otherwise the
+                         function name will be used.
         """
+
         if callable(name):
             func = name
-            name = func.__name__  # Use function name as default
+            name = func.__name__
-            self.add_template_filter(func, name=name)  # Register filter with Flask
+            self.add_template_filter(func, name=name)
-
+            return func  # ✅ return original function (not a wrapper)
-            @wraps(func)
-            def wrapper(*args, **kwargs):
-                return func(*args, **kwargs)
-
-            return wrapper
 
         def decorator(func: T_template_filter) -> T_template_filter:
-            self.add_template_filter(func, name=name)  # Register filter with Flask
+            self.add_template_filter(func, name=name)
-
+            return func  # ✅ return original function (not a wrapper)
-            @wraps(func)
-            def wrapper(*args, **kwargs):
-                return func(*args, **kwargs)
-
-            return wrapper
 
         return decorator
 

tests/test_template_filters.py

@@ -0,0 +1,24 @@
+import pytest
+from flask import Flask, render_template_string
+
+def test_template_filter_without_parentheses():
+    app = Flask(__name__)
+
+    @app.template_filter
+    def double(x):
+        return x * 2
+
+    with app.app_context():
+        output = render_template_string("{{ 2 | double }}")
+        assert output == "4"
+
+def test_template_filter_with_parentheses():
+    app = Flask(__name__)
+
+    @app.template_filter()
+    def triple(x):
+        return x * 3
+
+    with app.app_context():
+        output = render_template_string("{{ 3 | triple }}")
+        assert output == "9"