How do I fill na values in a column with the average of previous non-na and next non-na value in pandas?

Question

Raw table:

Column A
5
nan
nan
15

New table:

Column A
5
10
10
15

Answer 1

One option might be the following (using fillna twice (with options ffill and bfill) and then averaging them):

import pandas as pd
import numpy as np

df = pd.DataFrame({'x': [np.nan, 5, np.nan, np.nan, 15]})
filled_series = [df['x'].fillna(method=m) for m in ('ffill', 'bfill')]
print(pd.concat(filled_series, axis=1).mean(axis=1))
# 0     5.0
# 1     5.0
# 2    10.0
# 3    10.0
# 4    15.0

As you can see, this works even if nan happens at the beginning or at the end.