Partitioning comments by date

Question

I have a list of Comment objects comments. Each Comment has several properties, one of which is the date the comment was posted. I've sorted this list by date and there is at least 1 comment per date in a range.

I would like to loop over comments and for every date, create a new list of comments just for that day. However, the way I'm currently doing it feels very inefficient. Is there a more pythonic or efficient way to do this? (I've implemented a daterange(start_date, end_date) generator function according to this answer):

comment_arrays = [] #array of arrays
for d in date_range(date(2015,1,1), date(2016,1,1)):
    day_comments = []
    for c in comments:
        if c.date == d:
            day_comments.append(c)
        else if c.date > d:
            comment_arrays.append(day_comments)

Answer 1

I think this would a great situation to use itertools.groupby in.

datekey = lambda x: x.date
comments.sort(key=datekey)
date_comments = [list(group) for date, group in itertools.groupby(comments, datekey)]

This of course groups all your comments, not only the ones in your date range. I'd use filter to skip the ones out of your range if you can't make use of the extra days comments.

Answer 2

You are walking daterange one time and comments many times. Since the latter is, most likely, much longer than the former, it's gonna be more efficient to do the other way round. Besides, daterange is homogeneous so it can be accessed by index:

startdate=date(2015,1,1)
enddate=date(2016,1,1)
ndays=(enddate-startdate).days+1 #without +1 if the last date shall not be included
comments_by_day=[[] for _ in range(ndays)] 
for c in comments:
    i=c.date-startdate
    try: if i>=0: comments_by_day[i].append(c)  #avoid wraparound behavior
                                                # for negative indices
    except IndexError: pass   # date not in range