[Tutor] Input is Dictionary1, Output is Dictionary2 (using keys and values of Dictionary1)

[Mats Wichmann]

On 1/30/20 2:47 PM, Mats Wichmann wrote:
> On 1/30/20 1:23 PM, Panchanathan Suresh wrote:
>> Hi Everyone,
>>
>> I spent a lot of time on this, trying out many things, but I am unable to
>> create a new dictionary with the users as keys and a list of their groups
>> as values.
>>
>> Input Dictionary is {"local": ["admin", "userA"],"public":  ["admin",
>> "userB"],"administrator": ["admin"] }
>> Expected Output Dictionary is {"userA": ["admin","public"], "userB":
>> ["admin"], "administrator": ["admin"]}
>>
>> --- How to create an unique group for a particular user?
>> --- And then how to add this user key and the user's value (which will be
>> the list of groups the user belongs to)
>>
>> *****
>> def groups_per_user(group_dictionary):
>>         user_groups = {}
>>         groups = []
>>         # Go through group_dictionary
>>         for group,user in group_dictionary.items():
>>                 # Now go through the users in the group
>>                 for user1 in user:
>>                         groups.append(group)
>>                         print(user1,groups)
>>
>>         return(user_groups)
>>
>> print(groups_per_user({"local": ["admin", "userA"],"public":  ["admin",
>> "userB"],"administrator": ["admin"] }))
>>
>> *****
> 
> Hopefully this isn't a homework problem :)
> 
> You're not adding anything to your new dictionary. What you want to do
> is use the user as a key, and a list of groups as the value; the trick
> is that you may need to add several times to a user's entry, so you need
> to handle both the case of "user not yet in dictionary" and the case of
> "user has a grouplist, add to that list".  As a quick and dirty hack,
> like this (I changed a name a bit for clarity):
> 
>     for group, userlist in group_dictionary.items():
>         # Now go through the users in the group
>         for user in userlist:
>             try:
>                 user_groups[user].append(group)
>             except KeyError:
>                 user_groups[user] = [group]

Now some time has passed, I wanted to add, as a somewhat advanced topic
for those who may read this in the archives, that there are more concise
approaches. One way is to use, for the new calculated dictionary, a
derived class of dict, the defaultdict.  With defaultdict, the behavior
of accessing a key that doesn't exist changes: instead of raising a
KeyError, the key is automatically inserted with an empty value of the
type specified when instantiating the defaultdict, and then things
proceed as normal. In this case we want a list, so we can rewrite the
above stuff like this:

from collections import defaultdict

def groups_per_user(group_dictionary):
    user_groups = defaultdict(list)
    # Go through group_dictionary
    for group, userlist in group_dictionary.items():
        # Now go through the users in the group
        for user in userlist:
            user_groups[user].append(group)

    return user_groups