Can we sort a list following several attributes?

Question

So let's assume that we have this list:

data=[[A, 5, 200], [B, 5, 220], [C, 4, 150], [D, 4, 150], [E, 1, 50]]

and we want to sort it with the first number (increasing order) and in case of a tie, the second number (decreasing order), and in case of a tie again, in alphabetical order. I tried this code but it just didn't work:

data = sorted(data, key = operator.itemgetter(1, 2))

Is there a better way of doing this ?

Is it a list of sublists? Do not see the commas inside the inner square brackets — Valentino, Apr 30 '19 at 19:18
you seem to have illegal python - no commas, no `'` around strings, etc. Please prepare a [mcve] and what you did to solve it. — Patrick Artner, Apr 30 '19 at 19:20

chepner · Accepted Answer · 2019-04-30T19:29:41.353

1

In general, you need a key function that is easier to define using functools.cmp_to_key:

# Quick and dirty definition of cmp for Python 3
def cmp(a, b):
    """Return 0 if a == b, negative if a < b, and positive if a > b"""
    return (a > b) - (a < b)

def compare(a, b):
    return (cmp(a[1], b[1]) 
            or cmp(b[2], a[2])  # swap arguments for reversed ordering
            or cmp(a[0], b[0]))

sorted(data, key=functools.cmp_to_key(compare))

However, you can take advantage of the fact that for numbers, a < b implies -a > -b, to write

sorted(data, key=lambda a: (a[1], -a[2], a[0]))

edited Apr 30 '19 at 19:29

answered Apr 30 '19 at 19:20

chepner

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I fail to see how this will handle the tiebreak `lambda a, b: a[1] < b[1] or a[2] > b[2] or a[0] < a[1])`. The second solution works, though – Jean-François Fabre Apr 30 '19 at 19:22
Yeah, that needs some work. – chepner Apr 30 '19 at 19:24

Can we sort a list following several attributes?

1 Answers1