df.fillna(-999,inplace=True)
df2.replace(-999, np.nan, inplace=True)
df2.fillna(df2.mean())
EventId A B C
0 100000 0.91 124.711 2.666000
1 100001 0.91 124.711 -0.202838
2 100002 0.91 124.711 -0.202838
3 100003 0.91 124.711 -0.202838