Start a websocket for symbol market depth returning either a diff or a partial book

        https://github.com/binance-exchange/binance-official-api-docs/blob/master/web-socket-streams.md#partial-book-depth-streams

        :param symbol: required
        :type symbol: str
        :param callback: callback function to handle messages
        :type callback: function
        :param depth: optional Number of depth entries to return, default None. If passed returns a partial book instead of a diff
        :type depth: str

        :returns: connection key string if successful, False otherwise

        Partial Message Format

        .. code-block:: python

            {
                "lastUpdateId": 160,  # Last update ID
                "bids": [             # Bids to be updated
                    [
                        "0.0024",     # price level to be updated
                        "10",         # quantity
                        []            # ignore
                    ]
                ],
                "asks": [             # Asks to be updated
                    [
                        "0.0026",     # price level to be updated
                        "100",        # quantity
                        []            # ignore
                    ]
                ]
            }


        Diff Message Format

        .. code-block:: python

            {
                "e": "depthUpdate", # Event type
                "E": 123456789,     # Event time
                "s": "BNBBTC",      # Symbol
                "U": 157,           # First update ID in event
                "u": 160,           # Final update ID in event
                "b": [              # Bids to be updated
                    [
                        "0.0024",   # price level to be updated
                        "10",       # quantity
                        []          # ignore
                    ]
                ],
                "a": [              # Asks to be updated
                    [
                        "0.0026",   # price level to be updated
                        "100",      # quantity
                        []          # ignore
                    ]
                ]
            }

def start_depth_socket(self, symbol, callback, depth=None):
        """Start a websocket for symbol market depth returning either a diff or a partial book

        https://github.com/binance-exchange/binance-official-api-docs/blob/master/web-socket-streams.md#partial-book-depth-streams

        :param symbol: required
        :type symbol: str
        :param callback: callback function to handle messages
        :type callback: function
        :param depth: optional Number of depth entries to return, default None. If passed returns a partial book instead of a diff
        :type depth: str

        :returns: connection key string if successful, False otherwise

        Partial Message Format

        .. code-block:: python

            {
                "lastUpdateId": 160,  # Last update ID
                "bids": [             # Bids to be updated
                    [
                        "0.0024",     # price level to be updated
                        "10",         # quantity
                        []            # ignore
                    ]
                ],
                "asks": [             # Asks to be updated
                    [
                        "0.0026",     # price level to be updated
                        "100",        # quantity
                        []            # ignore
                    ]
                ]
            }


        Diff Message Format

        .. code-block:: python

            {
                "e": "depthUpdate", # Event type
                "E": 123456789,     # Event time
                "s": "BNBBTC",      # Symbol
                "U": 157,           # First update ID in event
                "u": 160,           # Final update ID in event
                "b": [              # Bids to be updated
                    [
                        "0.0024",   # price level to be updated
                        "10",       # quantity
                        []          # ignore
                    ]
                ],
                "a": [              # Asks to be updated
                    [
                        "0.0026",   # price level to be updated
                        "100",      # quantity
                        []          # ignore
                    ]
                ]
            }

        """
        socket_name = symbol.lower() + '@depth'
        if depth and depth != '1':
            socket_name = '{}{}'.format(socket_name, depth)
        return self._start_socket(socket_name, callback)