the comparison using `is not None` fails if the column `exit_tag` is stored with the more efficient dtype `Sparse[string]`. The reason is that none values are stored as np.nan which is a float. This would raise an error when `len` is called at the next condition.
```
2022-01-18 15:18:22,548 - freqtrade - ERROR - Fatal exception!
joblib.externals.loky.process_executor._RemoteTraceback:
"""
Traceback (most recent call last):
File "/home/ftuser/.local/lib/python3.9/site-packages/joblib/externals/loky/process_executor.py", line 436, in _process_worker
r = call_item()
File "/home/ftuser/.local/lib/python3.9/site-packages/joblib/externals/loky/process_executor.py", line 288, in __call__
return self.fn(*self.args, **self.kwargs)
File "/home/ftuser/.local/lib/python3.9/site-packages/joblib/_parallel_backends.py", line 595, in __call__
return self.func(*args, **kwargs)
File "/home/ftuser/.local/lib/python3.9/site-packages/joblib/parallel.py", line 262, in __call__
return [func(*args, **kwargs)
File "/home/ftuser/.local/lib/python3.9/site-packages/joblib/parallel.py", line 262, in <listcomp>
return [func(*args, **kwargs)
File "/home/ftuser/.local/lib/python3.9/site-packages/joblib/externals/loky/cloudpickle_wrapper.py", line 38, in __call__
return self._obj(*args, **kwargs)
File "/freqtrade/freqtrade/optimize/hyperopt.py", line 317, in generate_optimizer
bt_results = self.backtesting.backtest(
File "/freqtrade/freqtrade/optimize/backtesting.py", line 658, in backtest
trade_entry = self._get_sell_trade_entry(trade, row)
File "/freqtrade/freqtrade/optimize/backtesting.py", line 461, in _get_sell_trade_entry
return self._get_sell_trade_entry_for_candle(trade, sell_row)
File "/freqtrade/freqtrade/optimize/backtesting.py", line 426, in _get_sell_trade_entry_for_candle
and len(sell_row[EXIT_TAG_IDX]) > 0
TypeError: object of type 'float' has no len()
"""
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "/freqtrade/freqtrade/main.py", line 37, in main
return_code = args['func'](args)
File "/freqtrade/freqtrade/commands/optimize_commands.py", line 103, in start_hyperopt
hyperopt.start()
File "/freqtrade/freqtrade/optimize/hyperopt.py", line 482, in start
f_val = self.run_optimizer_parallel(parallel, asked, i)
File "/freqtrade/freqtrade/optimize/hyperopt.py", line 396, in run_optimizer_parallel
return parallel(delayed(
File "/home/ftuser/.local/lib/python3.9/site-packages/joblib/parallel.py", line 1056, in __call__
self.retrieve()
File "/home/ftuser/.local/lib/python3.9/site-packages/joblib/parallel.py", line 935, in retrieve
self._output.extend(job.get(timeout=self.timeout))
File "/home/ftuser/.local/lib/python3.9/site-packages/joblib/_parallel_backends.py", line 542, in wrap_future_result
return future.result(timeout=timeout)
File "/usr/local/lib/python3.9/concurrent/futures/_base.py", line 445, in result
return self.__get_result()
File "/usr/local/lib/python3.9/concurrent/futures/_base.py", line 390, in __get_result
raise self._exception
TypeError: object of type 'float' has no len()
ERROR: 1
```
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