Why itertools.groupby() doesn't work? [duplicate]

Answer 1

itertools uses the sort order of the data. Your list is not sorted.

So if you have ["gmail.com", "something.com", "gmail.com"] itertools will create three groups. This is different than the groupby in some functional languages (or Python pandas for that sake).

You need to sort the dict first.

import itertools

students = [{'name': 'Paul', 'mail': '@gmail.com'}, {'name': 'Tom',    'mail': '@yahoo.com'},
            {'name': 'Jim', 'mail': 'gmail.com'}, {'name': 'Jules', 'mail': '@something.com'},
            {'name': 'Gregory', 'mail': '@gmail.com'}, {'name': 'Kathrin', 'mail': '@something.com'}]


 for key, group in itertools.groupby(sorted(students, key=lambda x: x["mail"]), key=lambda student: student['mail']):
     print(key)
     print(list(group))

# @gmail.com
# [{'name': 'Paul', 'mail': '@gmail.com'}, {'name': 'Gregory', 'mail': '@gmail.com'}]
# @something.com
# [{'name': 'Jules', 'mail': '@something.com'}, {'name': 'Kathrin', 'mail': '@something.com'}]
# @yahoo.com
#[{'name': 'Tom', 'mail': '@yahoo.com'}]
#gmail.com
# [{'name': 'Jim', 'mail': 'gmail.com'}]