homepage

Consider this paragraph of PEP3333, referring to headers obtained
via start_response, emphasis mine:
 Instead, it must store them for the server or gateway to
 transmit only after the first iteration of the application
 return value that yields a *non-empty bytestring*, or upon
 the application's first invocation of the write() callable.
This means that an WSGI app such as this should be valid, because the
yielded bytes pre-start_response are empty:
 def application(environ, start_response):
 yield b''
 start_response("200 OK", [("Content-Type", "text/plain")])
 yield b'Hello, World.\n'
However, in wsgiref's simple server, this fails:
 Traceback (most recent call last):
 File "/usr/local/lib/python3.4/wsgiref/handlers.py", line 180, in finish_response
 self.write(data)
 File "/usr/local/lib/python3.4/wsgiref/handlers.py", line 269, in write
 raise AssertionError("write() before start_response()")
 AssertionError: write() before start_response()