How to replace the NaN values with the values from the first df:
country sex year cancer
0 Albania female 2000 32
1 Albania male 2000 58
2 Antigua female 2000 2
3 Antigua male 2000 5
4 Argen female 2000 591
5 Argen male 2000 2061
in the second df:
country year sex cancer
0 Albania 1985 female NaN
1 Albania 1985 male NaN
2 Albania 1986 female NaN
3 Albania 1986 male NaN
4 Albania 1987 female 25.0
5 Antigua 1992 male NaN
6 Antigua 1985 female NaN
the final should look like:
country year sex cancer
0 Albania 1985 female 32
1 Albania 1985 male 58
2 Albania 1986 female 32
3 Albania 1986 male 58
4 Albania 1987 female 25
5 Antigua 1992 male 5
6 Antigua 1985 female 2
Important are 2 conditions Country and Sex