# Buggy code
async def _async_get_candle_history(
    self, pair: str, timeframe: str, since_ms: Optional[int] = None
) -> Tuple[str, str, List]:
    """
    Asynchronously gets candle histories using fetch_ohlcv
    returns tuple: (pair, timeframe, ohlcv_list)
    """
    try:
        # fetch ohlcv asynchronously
        s = (
            "(" + arrow.get(since_ms // 1000).isoformat() + ") "
            if since_ms is not None
            else ""
        )
        logger.debug(
            "Fetching pair %s, interval %s, since %s %s...",
            pair,
            timeframe,
            since_ms,
            s,
        )

        data = await self._api_async.fetch_ohlcv(
            pair, timeframe=timeframe, since=since_ms
        )

        # Because some exchange sort Tickers ASC and other DESC.
        # Ex: Bittrex returns a list of tickers ASC (oldest first, newest last)
        # when GDAX returns a list of tickers DESC (newest first, oldest last)
        # Only sort if necessary to save computing time
        try:
            if data and data[0][0] > data[-1][0]:
                data = sorted(data, key=lambda x: x[0])
        except IndexError:
            logger.exception("Error loading %s. Result was %s.", pair, data)
            return pair, timeframe, []
        logger.debug("Done fetching pair %s, interval %s ...", pair, timeframe)
        return pair, timeframe, data

    except ccxt.NotSupported as e:
        raise OperationalException(
            f"Exchange {self._api.name} does not support fetching historical candlestick data."
            f"Message: {e}"
        ) from e
    except (ccxt.NetworkError, ccxt.ExchangeError) as e:
        raise TemporaryError(
            f"Could not load ticker history due to {e.__class__.__name__}. Message: {e}"
        ) from e
    except ccxt.BaseError as e:
        raise OperationalException(f"Could not fetch ticker data. Msg: {e}") from e


# Traceback (Exception)
2020-02-20 22:47:28,025 - freqtrade - ERROR - Fatal exception!
Traceback (most recent call last):
File "/opt/tradebot/freqtrade/.env/lib/python3.7/site-packages/talib/__init__.py", line 20, in wrapper
for arg in chain(args, kwargs.values())
StopIteration

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/freqtrade/freqtrade/main.py", line 36, in main
return_code = args['func'](args)
File "/freqtrade/freqtrade/commands/optimize_commands.py", line 76, in start_hyperopt
hyperopt.start()
File "/freqtrade/freqtrade/optimize/hyperopt.py", line 488, in start
preprocessed = self.backtesting.strategy.tickerdata_to_dataframe(data)
File "/freqtrade/freqtrade/strategy/interface.py", line 448, in tickerdata_to_dataframe
for pair, pair_data in tickerdata.items()}
File "/freqtrade/freqtrade/strategy/interface.py", line 448, in <dictcomp>
for pair, pair_data in tickerdata.items()}
File "/freqtrade/freqtrade/strategy/interface.py", line 464, in advise_indicators
return self.populate_indicators(dataframe, metadata)
File "/freqtrade/user_data/strategies/MyDev.py", line 205, in populate_indicators
dataframe['mfi'] = ta.MFI(dataframe)
File "talib/_abstract.pxi", line 352, in talib._ta_lib.Function.__call__
File "talib/_abstract.pxi", line 383, in talib._ta_lib.Function.__call_function
File "/opt/tradebot/freqtrade/.env/lib/python3.7/site-packages/talib/__init__.py", line 24, in wrapper
return func(*args, **kwargs)
File "talib/_func.pxi", line 8454, in talib._ta_lib.MFI
Exception: volume is not double

# Fixed code
async def _async_get_candle_history(
    self, pair: str, timeframe: str, since_ms: Optional[int] = None
) -> Tuple[str, str, List]:
    """
    Asynchronously gets candle histories using fetch_ohlcv
    returns tuple: (pair, timeframe, ohlcv_list)
    """
    try:
        # fetch ohlcv asynchronously
        s = (
            "(" + arrow.get(since_ms // 1000).isoformat() + ") "
            if since_ms is not None
            else ""
        )
        logger.debug(
            "Fetching pair %s, interval %s, since %s %s...",
            pair,
            timeframe,
            since_ms,
            s,
        )

        data = await self._api_async.fetch_ohlcv(
            pair, timeframe=timeframe, since=since_ms
        )

        # Because some exchange sort Tickers ASC and other DESC.
        # Ex: Bittrex returns a list of tickers ASC (oldest first, newest last)
        # when GDAX returns a list of tickers DESC (newest first, oldest last)
        # Only sort if necessary to save computing time
        try:
            if data and data[0][0] > data[-1][0]:
                data = sorted(data, key=lambda x: x[0])
        except IndexError:
            logger.exception("Error loading %s. Result was %s.", pair, data)
            return pair, timeframe, []
        logger.debug("Done fetching pair %s, interval %s ...", pair, timeframe)
        return pair, timeframe, data

    except ccxt.NotSupported as e:
        raise OperationalException(
            f"Exchange {self._api.name} does not support fetching historical candlestick data."
            f"Message: {e}"
        ) from e
    except (ccxt.NetworkError, ccxt.ExchangeError) as e:
        raise TemporaryError(
            f"Could not load ticker history for pair {pair} due to "
            f"{e.__class__.__name__}. Message: {e}"
        ) from e
    except ccxt.BaseError as e:
        raise OperationalException(
            f"Could not fetch ticker data for pair {pair}. Msg: {e}"
        ) from e