def getHeader(key):
        """
        Get an HTTP request header.

        @type key: L{bytes} or L{str}
        @param key: The name of the header to get the value of.

        @rtype: L{bytes} or L{str} or L{None}
        @return: The value of the specified header, or L{None} if that header
            was not present in the request. The string type of the result
            matches the type of C{key}.
        """

        Get an HTTP request header.

        @type key: L{bytes} or L{str}
        @param key: The name of the header to get the value of.

        @rtype: L{bytes} or L{str} or L{None}
        @return: The value of the specified header, or L{None} if that header
            was not present in the request. The string type of the result
            matches the type of C{key}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/iweb.py