我认为您需要
map按列
A：
df['C'] = df.B.map(df.A)print (df) A B CIndex 2001-06-30 100 2001-08-31 300.02001-07-31 200 2001-09-30 400.02001-08-31 300 2001-10-31 NaN2001-09-30 400 2001-11-30 NaN
与以下内容相同：
df['C'] = df.B.map(df.A.to_dict())print (df) A B CIndex 2001-06-30 100 2001-08-31 300.02001-07-31 200 2001-09-30 400.02001-08-31 300 2001-10-31 NaN2001-09-30 400 2001-11-30 NaN
