Complex python list processing

Question

I have a python list made up of usernames and time stamp tuples. Imagine it's like the following:

[(username,datetime_obj),(username,datetime_obj),(username,datetime_obj),(username,datetime_obj),(username,datetime_obj),(username,datetime_obj),(username,datetime_obj),(username,datetime_obj),(username,datetime_obj)]

Next, imagine the list above has only 3 unique usernames, but that all datetime objects are unique.

What's the most efficient, pythonic way to derive a new list from the one above, which is again made up of tuples and the same usernames, except that next to each username, the most recent datetime_obj in the list (for that particular username) is repeatedly attached.

E.g. if starting list was [(sam,1),(sam,7),(sam,8),(jon,4),(mel,9),(mel,2),(mel,10),(jon,3),(jon,6)], I end up with [(sam,1),(sam,1),(sam,1),(jon,3),(mel,2),(mel,2),(mel,2),(jon,3),(jon,3)].

I used ints to depict datetime objects in the example above. This was just for simplicity.

Thanks in advance.

Just do the job with normal Python syntax (especially list comprehensions), and don't worry about efficiency or 'pythonic'. If you show code that works, we can suggest improvements. — hpaulj
– hpaulj, Commented Feb 4, 2016 at 3:09

L3viathan · Accepted Answer · 2016-02-04 03:15:17Z

3

I think you can't get around iterating over the list twice:

most_recent = {}
for user, date in myList:
    most_recent[user] = max(most_recent.get(user, date), date)

newList = [(user, most_recent[user]) for user, _ in myList]

You can do something like this, if you consider this more pythonic, but it is slower (quadratic complexity), so don't actually do it:

[(user, max(date for u, date in myList if u == user)) for user, _ in myList]

edited Feb 4, 2016 at 3:15

answered Feb 4, 2016 at 2:54

L3viathan

27.5k2 gold badges63 silver badges84 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

midori Over a year ago

i guess it should be [(user, min(date for u, date in myList if u == user)) for user, _ in myList]

L3viathan Over a year ago

@minitoto If they want the output described in the question, yes. But if they want the most recent date object, no. In any case, this is simple to change.

midori Over a year ago

yeah i understand that, OP simplified the I/O but it's confusing :)

ShadowRanger Over a year ago

It's not more Pythonic to use a list comp when you'd be nesting comprehensions excessively and turning O(n) algorithms into O(n**2) algorithms. My only changes would be: 1. Use {} to initialize most_recent instead of the constructor (trivial) and 2. Change the if and set to the one-line most_recent[user] = max(most_recent.get(user, date), date) to make the code avoid special casing the "first time seen" case and avoiding the need to pick a specific sentinel value (0 in this case, but arbitrary datetime harder to choose).

L3viathan Over a year ago

@ShadowRanger To me the list comprehension is slightly more readable, because it is a more direct translation of the semantics, but of course it should not be used. I agree with your other comments, and made changes accordingly.

Collectives™ on Stack Overflow

Complex python list processing

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related