Re: Sorting dictionary by 'sub' value

[Steven Bethard]

Rory Campbell-Lange wrote:
Thank you all very much for your help.
I did the following and it works:
    imgs=v.keys()
    imgs.sort(lambda a,b: cmp(
              time.strptime(str(v[a][9]['date']), '%Y:%m:%d %H:%M:%S'),
              time.strptime(str(v[b][9]['date']), '%Y:%m:%d %H:%M:%S'))
             )
    for i in imgs:
        ...
Cool.  If you ever find that this is a speed problem, it's worth 
pointing out that the decorate-sort-undecorate pattern is usually 
slightly faster:

------------------------------ test.py ------------------------------
import datetime
import time
# returns data in a similar format to yours
def get_data(n):
    today = datetime.datetime(2005, 3, 8)
    deltas = [datetime.timedelta(seconds=1e6*i)
              for i in xrange(-n, n)]
    times = [(today + delta).strftime('%Y:%m:%d %H:%M:%S')
             for delta in deltas]
    return dict([(i, {9:{'date':time}})
                 for i, time in enumerate(times)])
def sortcmp(data):
    imgs = data.keys()
    imgs.sort(lambda a,b: cmp(
              time.strptime(str(data[a][9]['date']),
                            '%Y:%m:%d %H:%M:%S'),
              time.strptime(str(data[b][9]['date']),
                            '%Y:%m:%d %H:%M:%S'))
             )
    return imgs
def sortdsu(data):
    decorated = [(time.strptime(str(data[key][9]['date']),
                                '%Y:%m:%d %H:%M:%S'), key)
                 for key in data]
    decorated.sort()
    return [key for date, key in decorated]
# Requires 2.4
def sortkey(data):
    def value(key):
        return time.strptime(str(data[key][9]['date']),
                             '%Y:%m:%d %H:%M:%S')
    return sorted(data, key=value)
---------------------------------------------------------------------
And the timing results:
[D:\Steve]$ python -m timeit -s "import test; d = test.get_data(1000)" 
"test.sortcmp(d)"
10 loops, best of 3: 274 msec per loop

[D:\Steve]$ python -m timeit -s "import test; d = test.get_data(1000)" 
"test.sortdsu(d)"
10 loops, best of 3: 131 msec per loop

# Requires 2.4
[D:\Steve]$ python -m timeit -s "import test; d = test.get_data(1000)" 
"test.sortkey(d)"
10 loops, best of 3: 131 msec per loop

HTH,
STeVe
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