如果 df2 中的单个列包含来自 df1 中的列的 value2，则在 df1 中创建具有 value1 的新 col4

Question

df1 for example is

col1	col2	col3
abcdef	ghijkl	mnopqr
abcdef1	ghijkl1	mnopqr1

df2 is

col1
ghijkl1

essentially I want to add a col4 to df1 with the value "MM" if the value in df1col2 appears in df2col1

the final df1 would be:

col1	col2	col3	col4
abcdef	ghijkl	mnopqr
abcdef1	ghijkl1	mnopqr1	MM

Answer 1

Use Series.isin and then chain .map to convert True to 'MM', and False to a NaN value.

df1['col4'] = df1['col2'].isin(df2['col1']).map({True:'MM',False:np.nan})

print(df1)

      col1     col2     col3 col4
0   abcdef   ghijkl   mnopqr  NaN
1  abcdef1  ghijkl1  mnopqr1   MM