for i in range(len(a)):
train[train.enrollee_id==a[i]]['enrolled_university']=train[train.enrollee_id==a[i]]['enrolled_university'].fillna(y_pred[I],inlace=True)
print(train[train.enrollee_id==a[i]]['enrolled_university’])
112 NaN
Name: enrolled_university, dtype: float64
191 NaN
Name: enrolled_university, dtype: float64
244 NaN
Name: enrolled_university, dtype: float64
284 NaN
Name: enrolled_university, dtype: float64
315 NaN
Name: enrolled_university, dtype: float64
401 NaN
Name: enrolled_university, dtype: float64