Concat

concat.txt

>>> data = [{'col1': 'a', 'col2': 'z'}, {'col1':'b', 'col2': 'z'}, {'col1':'c', 'col2':'y'}, {'col1':'d', 'col2':'y'}]

>>> data
[{'col1': 'a', 'col2': 'z'}, {'col1': 'b', 'col2': 'z'}, {'col1': 'c', 'col2': 'y'}, {'col1': 'd', 'col2': 'y'}]

>>> df = pandas.DataFrame(data)

>>> df.groupby('col2')['col1'].apply(lambda x : x.str.cat(sep=', '))
col2
y    c, d
z    a, b
Name: col1, dtype: object

>>>