docs: def distance_matrix(client, origins, destinations,
                      mode=None, language=None, avoid=None, units=None,
                      departure_time=None, arrival_time=None, transit_mode=None,
                      transit_routing_preference=None, traffic_model=None, region=None):
      """ Gets travel distance and time for a matrix of origins and destinations.
  
      :param origins: One or more locations and/or latitude/longitude values,
          from which to calculate distance and time. If you pass an address as
          a string, the service will geocode the string and convert it to a
          latitude/longitude coordinate to calculate directions.
      :type origins: a single location, or a list of locations, where a
          location is a string, dict, list, or tuple
  
      :param destinations: One or more addresses and/or lat/lng values, to
          which to calculate distance and time. If you pass an address as a
          string, the service will geocode the string and convert it to a
          latitude/longitude coordinate to calculate directions.
      :type destinations: a single location, or a list of locations, where a
          location is a string, dict, list, or tuple
  
      :param mode: Specifies the mode of transport to use when calculating
          directions. Valid values are "driving", "walking", "transit" or
          "bicycling".
      :type mode: string
  
      :param language: The language in which to return results.
      :type language: string
  
      :param avoid: Indicates that the calculated route(s) should avoid the
          indicated features. Valid values are "tolls", "highways" or "ferries".
      :type avoid: string
  
      :param units: Specifies the unit system to use when displaying results.
          Valid values are "metric" or "imperial".
      :type units: string
  
      :param departure_time: Specifies the desired time of departure.
      :type departure_time: int or datetime.datetime
  
      :param arrival_time: Specifies the desired time of arrival for transit
          directions. Note: you can't specify both departure_time and
          arrival_time.
      :type arrival_time: int or datetime.datetime
  
      :param transit_mode: Specifies one or more preferred modes of transit.
          This parameter may only be specified for requests where the mode is
          transit. Valid values are "bus", "subway", "train", "tram", "rail".
          "rail" is equivalent to ["train", "tram", "subway"].
      :type transit_mode: string or list of strings
  
      :param transit_routing_preference: Specifies preferences for transit
          requests. Valid values are "less_walking" or "fewer_transfers".
      :type transit_routing_preference: string
  
      :param traffic_model: Specifies the predictive travel time model to use.
          Valid values are "best_guess" or "optimistic" or "pessimistic".
          The traffic_model parameter may only be specified for requests where
          the travel mode is driving, and where the request includes a
          departure_time.
  
      :param region: Specifies the prefered region the geocoder should search
          first, but it will not restrict the results to only this region. Valid
          values are a ccTLD code.
      :type region: string
  
      :rtype: matrix of distances. Results are returned in rows, each row
          containing one origin paired with each destination.
      """
  
      params = {
          "origins": convert.location_list(origins),
          "destinations": convert.location_list(destinations)
      }
  
      if mode:
          # NOTE(broady): the mode parameter is not validated by the Maps API
          # server. Check here to prevent silent failures.
          if mode not in ["driving", "walking", "bicycling", "transit"]:
              raise ValueError("Invalid travel mode.")
          params["mode"] = mode
  
      if language:
          params["language"] = language
  
      if avoid:
          if avoid not in ["tolls", "highways", "ferries"]:
              raise ValueError("Invalid route restriction.")
          params["avoid"] = avoid
  
      if units:
          params["units"] = units
  
      if departure_time:
          params["departure_time"] = convert.time(departure_time)
  
      if arrival_time:
          params["arrival_time"] = convert.time(arrival_time)
  
      if departure_time and arrival_time:
          raise ValueError("Should not specify both departure_time and"
                           "arrival_time.")
  
      if transit_mode:
          params["transit_mode"] = convert.join_list("|", transit_mode)
  
      if transit_routing_preference:
          params["transit_routing_preference"] = transit_routing_preference
  
      if traffic_model:
          params["traffic_model"] = traffic_model
  
      if region:
          params["region"] = region
  
      return client._request("/maps/api/distancematrix/json", params)