>>> s = pd.Series([None, 3, 4])
>>> s.first_valid_index()
1
>>> s = pd.Series([None, None])
>>> s.first_valid_index()
>>> df = pd.DataFrame({'A': [None, 1, 2, None], 'B': [3, 2, 1, None]}, index=[10, 11, 12, 13])
>>> df
A B
10 NaN 3.0
11 1.0 2.0
12 2.0 1.0
13 NaN NaN
>>> df.first_valid_index()
10
>>> df = pd.DataFrame([5, 6, 7, 8], index=["i", "am", "iron", "man"])
>>> df.first_valid_index()
'i'