One to many mapping with a ChainMap Dictionary

I would like do a one to many mapping with the following list and mapping dictonary:

l1 = ['a', 'b', 'c']
l2 = ['a', 'c', 'd']
l3 = ['d', 'e', 'f']


mapping_dict = ChainMap(
        dict.fromkeys(l1, 'A'),
        dict.fromkeys(l2, 'B'),
        dict.fromkeys(l3, 'C'))

This is my dataframe:

df = pd.DataFrame({'code': ['a', 'b', 'c', 'd', 'e', 'f'], 'value': [1, 2, 3, 4, 5, 6]})
print(df)

  code  value
0    a      1
1    b      2
2    c      3
3    d      4
4    e      5
5    f      6

When I do the mapping as follows:

df['mapping'] = df['code'].map(mapping_dict.get)

  code  value mapping
0    a      1       A
1    b      2       A
2    c      3       A
3    d      4       B
4    e      5       C
5    f      6       C

The problem is that I want to do a one to many mapping and I don’t capture the relationship. The desired outcome would be something like this. Which creates a new line when there are multiple relationships.

  code  value mapping
0    a      1       A
1    a      1       B
2    b      2       A
3    c      3       A
4    c      3       B
5    d      4       B
6    d      4       C
7    e      5       C
8    e      5       C
9    f      6       C

Thank you for your support.

Answer

Here ChainMap can’t be used since it will not preserve all the duplicate keys. The solution is to create an intermediate dataframe from each pairs of (mapping, code) and then left merge that with the original dataframe

pairs = [('A', l1), ('B', l2), ('C', l3)]
mapping = pd.DataFrame(pairs, columns=['mapping', 'code'])

df.merge(mapping.explode('code'), how='left')

Result

  code  value mapping
0    a      1       A
1    a      1       B
2    b      2       A
3    c      3       A
4    c      3       B
5    d      4       B
6    d      4       C
7    e      5       C
8    f      6       C