feat: support call template_filter without parens

src/flask/sansio/app.py

@@ -662,8 +662,8 @@ class App(Scaffold):
 
     @setupmethod
     def template_filter(
-        self, name: str | None = None
+        self, name: t.Callable[..., t.Any] | str | None = None
-    ) -> t.Callable[[T_template_filter], T_template_filter]:
+    ) -> t.Callable[[T_template_filter], 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::
@@ -672,10 +672,26 @@ class App(Scaffold):
             def reverse(s):
                 return s[::-1]
 
+        The decorator also can be used without parentheses::
+
+            @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):
+            # If name is callable, it is the function to register.
+            # This is a shortcut for the common case of
+            # @app.template_filter
+            # def func():
+
+            func = name
+            self.add_template_filter(func)
+            return func
+
         def decorator(f: T_template_filter) -> T_template_filter:
             self.add_template_filter(f, name=name)
             return f

tests/test_templating.py

@@ -129,6 +129,30 @@ def test_template_filter(app):
     assert app.jinja_env.filters["my_reverse"] == my_reverse
     assert app.jinja_env.filters["my_reverse"]("abcd") == "dcba"
 
+    @app.template_filter
+    def my_reverse_2(s):
+        return s[::-1]
+
+    assert "my_reverse_2" in app.jinja_env.filters.keys()
+    assert app.jinja_env.filters["my_reverse_2"] == my_reverse_2
+    assert app.jinja_env.filters["my_reverse_2"]("abcd") == "dcba"
+
+    @app.template_filter("my_reverse_custom_name")
+    def my_reverse_3(s):
+        return s[::-1]
+
+    assert "my_reverse_custom_name" in app.jinja_env.filters.keys()
+    assert app.jinja_env.filters["my_reverse_custom_name"] == my_reverse_3
+    assert app.jinja_env.filters["my_reverse_custom_name"]("abcd") == "dcba"
+
+    @app.template_filter(name="my_reverse_custom_name_2")
+    def my_reverse_4(s):
+        return s[::-1]
+
+    assert "my_reverse_custom_name_2" in app.jinja_env.filters.keys()
+    assert app.jinja_env.filters["my_reverse_custom_name_2"] == my_reverse_4
+    assert app.jinja_env.filters["my_reverse_custom_name_2"]("abcd") == "dcba"
+
 
 def test_add_template_filter(app):
     def my_reverse(s):