From 6ff24afee826daf4348b1f04911dc51120b8827b Mon Sep 17 00:00:00 2001
From: Akshay <akshaychoudhary2040@gmail.com>
Date: Fri, 16 May 2025 14:49:45 +0530
Subject: [PATCH] Fix template_filter decorator to support both usage styles
 and update docstring; update CHANGES.rst

---
 CHANGES.rst                    |  2 ++
 src/flask/sansio/app.py        | 39 ++++++++++++++--------------------
 tests/test_template_filters.py | 24 +++++++++++++++++++++
 3 files changed, 42 insertions(+), 23 deletions(-)
 create mode 100644 tests/test_template_filters.py

diff --git a/CHANGES.rst b/CHANGES.rst
index 37e777dc..cf8b7a6f 100644
--- a/CHANGES.rst
+++ b/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
 -------------
diff --git a/src/flask/sansio/app.py b/src/flask/sansio/app.py
index 9c85d234..b4bb6639 100644
--- a/src/flask/sansio/app.py
+++ b/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.
-        You can specify a name for the filter, otherwise the function
-        name will be used. Example::
+        """A decorator that is used to register custom template filters.
+              You can use this with or without parentheses. Example::
 
-          @app.template_filter()
-          def reverse(s):
-              return s[::-1]
+              @app.template_filter
+              def double(x):
+                  return x * 2
 
-        :param name: the optional name of the filter, otherwise the
-                     function name will be used.
+              @app.template_filter()
+              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
-            self.add_template_filter(func, name=name)  # Register filter with Flask
-
-            @wraps(func)
-            def wrapper(*args, **kwargs):
-                return func(*args, **kwargs)
-
-            return wrapper
+            name = func.__name__
+            self.add_template_filter(func, name=name)
+            return func  # ✅ return original function (not a wrapper)
 
         def decorator(func: T_template_filter) -> T_template_filter:
-            self.add_template_filter(func, name=name)  # Register filter with Flask
-
-            @wraps(func)
-            def wrapper(*args, **kwargs):
-                return func(*args, **kwargs)
-
-            return wrapper
+            self.add_template_filter(func, name=name)
+            return func  # ✅ return original function (not a wrapper)
 
         return decorator
 
diff --git a/tests/test_template_filters.py b/tests/test_template_filters.py
new file mode 100644
index 00000000..7dda15f8
--- /dev/null
+++ b/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"