Don't set headers for responses with 1xx, 204 and 304 status code

starlette/responses.py

@@ -71,7 +71,11 @@ def init_headers(self, headers: typing.Mapping[str, str] = None) -> None:
             populate_content_type = b"content-type" not in keys
 
         body = getattr(self, "body", None)
-        if body is not None and populate_content_length:
+        if (
+            body is not None
+            and populate_content_length
+            and not (self.status_code < 200 or self.status_code in (204, 304))
+        ):
             content_length = str(len(body))
             raw_headers.append((b"content-length", content_length.encode("latin-1")))
 

tests/test_responses.py

@@ -1,4 +1,5 @@
 import os
+from typing import Callable
 
 import anyio
 import pytest
@@ -366,3 +367,20 @@ def test_streaming_response_known_size(test_client_factory):
     client: TestClient = test_client_factory(app)
     response = client.get("/")
     assert response.headers["content-length"] == "10"
+
+
+@pytest.mark.parametrize("status_code", (100, 204, 304))
+def test_no_content_length(
+    status_code: int, test_client_factory: Callable[..., TestClient]
+):
+    app = Response(content=b"non_empty", status_code=status_code)
+    client = test_client_factory(app)
+    response = client.get("/")
+    assert "content-length" not in response.headers
+
+
+def test_json_response_no_content_length(test_client_factory):
+    app = JSONResponse("null", status_code=204)
+    client = test_client_factory(app)
+    response = client.get("/")
+    assert "content-length" not in response.headers