pandas.Series.first_valid_index
- 
Series.first_valid_index()[source] - 
Return index for first non-NA/null value.
Returns: - 
scalar : type of index 
Notes
If all elements are non-NA/null, returns None. Also returns None for empty NDFrame.
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    https://pandas.pydata.org/pandas-docs/version/0.23.4/generated/pandas.Series.first_valid_index.html