Fill missing row values as an average of previous and next row in pandas dataframe

Question

My data frame looks like this

            county_name     state       year    rank county_population  city_population
31          Fairfax County  Virginia    2010.0  0.0   1086730.0         60300
32          Fairfax County  Virginia    2011.0  0.0   1099603.0         60300
33          Fairfax County  Virginia    2013.0  0.0   1130364.0         60300
34          Fairfax County  Virginia    2014.0  0.0   1138123.0         60300
35          Fairfax County  Virginia    2015.0  0.0   1142245.0         60300

I want to insert the missing row for year 2012 and assign it rank 7. For the values of county and city population, I want to take an average of the previous and next rows (2011 and 2013) and fill those values for the missing row.

Any pointers will be highly appreciated

EDIT 1: Expected data frame should be

            county_name     state       year    rank county_population  city_population
31          Fairfax County  Virginia    2010.0  0.0   1086730.0         60300
32          Fairfax County  Virginia    2011.0  0.0   1099603.0         60300
33          Fairfax County  Virginia    2012.0  7.0   1114984.0         60300
34          Fairfax County  Virginia    2013.0  0.0   1130364.0         60300
35          Fairfax County  Virginia    2014.0  0.0   1138123.0         60300
36          Fairfax County  Virginia    2015.0  0.0   1142245.0         60300

Answer 1

Create a new dataframe and merge them, sort by year and interpolate missing values:

data = [['Fairfax County', 'Virginia', 2012, 7, np.NaN, np.NaN]]
out = df.append(pd.DataFrame(data, columns=df.columns)) \
        .sort_values('year').interpolate()
print(out)

Output result:

>>> out
       county_name     state  year  rank  county_population  city_population
31  Fairfax County  Virginia  2010   0.0          1086730.0          60300.0
32  Fairfax County  Virginia  2011   0.0          1099603.0          60300.0
0   Fairfax County  Virginia  2012   7.0          1114983.5          60300.0
33  Fairfax County  Virginia  2013   0.0          1130364.0          60300.0
34  Fairfax County  Virginia  2014   0.0          1138123.0          60300.0
35  Fairfax County  Virginia  2015   0.0          1142245.0          60300.0