name, userId, email
john, null, john@a.com
alex, null, alex@a.com
micheal, null, mike@a.com
alex, null, alex@a.com
john, null, john@a.com

name, userId, email
alex, 5, alex@a.com
micheal, 10, mike@a.com
john, 12, john@a.com

name, userId, email
john, 12, john@a.com
alex, 5, alex@a.com
micheal, 10, mike@a.com
alex, 5, alex@a.com
john, 12, john@a.com

import pandas as pd

df1 = pd.read_csv("file1.csv", sep=",")
df2 = pd.read_csv("file2.csv", sep=",", index_col=0)

df1["userId"] = df2["userId"].values

df1.to_csv("output.csv", sep=";")