drop_duplicates与参数一起使用，
subset仅保留最后重复的行add
keep='last'：
df1 = df.drop_duplicates(subset=['A','B'])#same as#df1 = df.drop_duplicates(subset=['A','B'], keep='first')print (df1) A B C0 1 2 x2 3 4 z3 3 5 x
df2 = df.drop_duplicates(subset=['A','B'], keep='last')print (df2) A B C1 1 2 y2 3 4 z3 3 5 x
