Deprecate changing the status code of a response after it has been prepared

CHANGES.rst

@@ -52,6 +52,9 @@ CHANGES
 - Avoid a race when application might start accepting incoming requests
   but startup signals are not processed yet e98e8c6
 
+- Raise a `RuntimeError` when trying to change the status of the HTTP response
+  after the headers have been sent
+
 -
 
 - Fix bug with https proxy acquired cleanup #1340

aiohttp/web_reqrep.py

@@ -509,7 +509,6 @@ def __init__(self, *, status=200, reason=None, headers=None):
         self._compression_force = False
         self._headers = CIMultiDict()
         self._cookies = SimpleCookie()
-        self.set_status(status, reason)
 
         self._req = None
         self._resp_impl = None
@@ -522,6 +521,8 @@ def __init__(self, *, status=200, reason=None, headers=None):
             self._headers.extend(headers)
         self._headers.setdefault(hdrs.CONTENT_TYPE, 'application/octet-stream')
 
+        self.set_status(status, reason)
+
     @property
     def prepared(self):
         return self._resp_impl is not None
@@ -552,6 +553,9 @@ def reason(self):
         return self._reason
 
     def set_status(self, status, reason=None):
+        if self.prepared:
+            raise RuntimeError("Cannot change the response status code after "
+                               "the headers have been sent")
         self._status = int(status)
         if reason is None:
             reason = ResponseImpl.calc_reason(status)

tests/test_web_response.py

@@ -905,6 +905,14 @@ def test_drain_before_start():
         yield from resp.drain()
 
 
+@asyncio.coroutine
+def test_changing_status_after_prepare_raises():
+    resp = StreamResponse()
+    yield from resp.prepare(make_request('GET', '/'))
+    with pytest.raises(RuntimeError):
+        resp.set_status(400)
+
+
 def test_nonstr_text_in_ctor():
     with pytest.raises(TypeError):
         Response(text=b'data')