Compare values in list of dicts in Python

Question

I'm a newbie in Python. I have a list of members and a list of meetings (containing the member id):

memberList = [{'id': '1', 'name': 'Joe'},
{'id': '2', 'name': 'Jason'},
{'id': '3', 'name': 'Billy'}]

meetingList = [{'meetingId': '20', 'hostId' : '1'},
{'meetingId': '21', 'hostId' : '1'},
{'meetingId': '22', 'hostId' : '2'},
{'meetingId': '23', 'hostId' : '2'}]

Where the id of the member and the hostId of meeting is the same value.

Result: a list of member ids which has no meetings ['3'] or the list of dicts [{'id': '3', 'name': 'Billy'}]

What's the best and most readable way to do it?

Answer 1

You could build a set of hosts and then use a list comprehension to filter out the members:

member_list = [{'id': '1', 'name': 'Joe'},
               {'id': '2', 'name': 'Jason'},
               {'id': '3', 'name': 'Billy'}]

meeting_list = [{'meetingId': '20', 'hostId': '1'},
                {'meetingId': '21', 'hostId': '1'},
                {'meetingId': '22', 'hostId': '2'},
                {'meetingId': '23', 'hostId': '2'}]

# create a set of hosts
hosts = set(meeting['hostId'] for meeting in meeting_list)  # or { meeting['hostId'] for meeting in meeting_list }

# filter out the members that are in hosts
res = [member['id'] for member in member_list if member['id'] not in hosts]
print(res)

Output

[{'id': '3', 'name': 'Billy'}]

For the id only output, do:

res = [member['id'] for member in member_list if member['id'] not in hosts]
print(res)

Output

['3']