code: def _init_length(self, request_method):
          
          length = self.headers.get('content-length')
  
          if length is not None:
              if self.chunked:
                  # This Response will fail with an IncompleteRead if it can't be
                  # received as chunked. This method falls back to attempt reading
                  # the response before raising an exception.
                  log.warning("Received response with both Content-Length and "
                              "Transfer-Encoding set. This is expressly forbidden "
                              "by RFC 7230 sec 3.3.2. Ignoring Content-Length and "
                              "attempting to process response as Transfer-Encoding: "
                              "chunked.")
                  return None
  
              try:
                  # RFC 7230 section 3.3.2 specifies multiple content lengths can
                  # be sent in a single Content-Length header
                  # (e.g. Content-Length: 42, 42). This line ensures the values
                  # are all valid ints and that as long as the `set` length is 1,
                  # all values are the same. Otherwise, the header is invalid.
                  lengths = set([int(val) for val in length.split(',')])
                  if len(lengths) > 1:
                      raise InvalidHeader("Content-Length contained multiple "
                                          "unmatching values (%s)" % length)
                  length = lengths.pop()
              except ValueError:
                  length = None
              else:
                  if length < 0:
                      length = None
  
          # Convert status to int for comparison
          # In some cases, httplib returns a status of "_UNKNOWN"
          try:
              status = int(self.status)
          except ValueError:
              status = 0
  
          # Check for responses that shouldn't include a body
          if status in (204, 304) or 100 <= status < 200 or request_method == 'HEAD':
              length = 0
  
          return length
docs: Set initial length value for Response content if available.