Add Headers.mutablecopy()

starlette/datastructures.py

@@ -155,6 +155,9 @@ class Headers(typing.Mapping[str, str]):
             if item_key == get_header_key
         ]
 
+    def mutablecopy(self):
+        return MutableHeaders(self._list[:])
+
     def __getitem__(self, key: str):
         get_header_key = key.lower().encode("latin-1")
         for header_key, header_value in self._list:

tests/test_datastructures.py

@@ -51,6 +51,13 @@ def test_mutable_headers():
     assert dict(h) == {"b": "4"}
 
 
+def test_headers_mutablecopy():
+    h = Headers([(b"a", b"123"), (b"a", b"456"), (b"b", b"789")])
+    c = h.mutablecopy()
+    assert c.items() == [("a", "123"), ("a", "456"), ("b", "789")]
+    c["a"] = "abc"
+    assert c.items() == [("b", "789"), ("a", "abc")]
+
 def test_queryparams():
     q = QueryParams([("a", "123"), ("a", "456"), ("b", "789")])
     assert "a" in q