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CodePudding user response：
As the documentation states, to_csv return None if you supply the path_or_buf parameter (which you did, it's the first argument). So pandas act as it supposes, there is no problem.
If you want to print the csv (and don't save it) you can:
print(forecast_array.to_csv(sep=';'))
Not supplying `path_or_buf' parameter, this will return the csv formatted as a string
