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

…epared

aiohttp/web_reqrep.py

@@ -456,7 +456,6 @@ def __init__(self, *, status=200, reason=None, headers=None):
         self._compression_force = False
         self._headers = CIMultiDict()
         self._cookies = http.cookies.SimpleCookie()
-        self.set_status(status, reason)
 
         self._req = None
         self._resp_impl = None
@@ -466,6 +465,8 @@ def __init__(self, *, status=200, reason=None, headers=None):
             self._headers.extend(headers)
         if hdrs.CONTENT_TYPE not in self._headers:
             self._headers[hdrs.CONTENT_TYPE] = 'application/octet-stream'
+
+        self.set_status(status, reason)
 
     def _copy_cookies(self):
         for cookie in self._cookies.values():
@@ -498,6 +499,8 @@ def reason(self):
         return self._reason
 
     def set_status(self, status, reason=None):
+        if self.prepared:
+            warnings.warn('Changing the response status after it has been started is a no-op and will be ignored', DeprecationWarning)
         self._status = int(status)
         if reason is None:
             reason = ResponseImpl.calc_reason(status)