Question

I have a list of lists of three strings in Python, e.g.,

m = [['pig', 'quartz', '1'], ['pork', 'nails', '1'], ...]

and I want to sort it by index 0, then by index 2 in reverse order, then by index 1. At each step in the sort I’d like to maintain the order imposed by the other columns. For instance,

pork       barn         4
pork       barn2        4
pork       nails        1
pig        quartz       1
quinoa     pail         1
quinoa     quatern      1
quail      quatern      1
radish     barn         1
radish     barn2        1
radish     inbox        3
radish     snow         1

would become:

pig        quartz       1
pork       barn         4
pork       barn2        4
pork       nails        1
quail      quatern      1
quinoa     pail         1
quinoa     quatern      1
radish     inbox        3    <-
radish     barn         1
radish     barn2        1
radish     snow         1

that is, sort by the first column, then within each group of 1st columns (pig, pork, quail, …), sort by the third column reversed, then within each group of 1st-column-3rd-column ((pig, 1), (pork, 4), (pork, 1), …), sort by the second column.

How can I do this nicely? Conceptually, if operator.itemgetter() could encode sort order along with index, I would want something like m.sort(key=operator.itemgetter(0, -2, 1)).

Answer 1

Use a custom comparison function:

def compare_triples(a, b):
    ret=cmp(a[0], b[0])
    if ret: return ret
    ret=cmp(b[2], a[2])
    if ret: return ret
    return cmp(a[1], b[1])

for i in m: print i
print "-" * 79
m2=sorted(m, cmp=compare_triples)
for i in m2: print i

The compare function is not optimal and might be rewritten as:

def compare_triples(a, b):
    return cmp((a[0], b[2], a[1]), (b[0], a[2], b[1]))

As others have pointed out this will work too:

def sort_key(a):
    return a[0], -int(a[2]), a[1]

for i in sorted(m, key=sort_key): print i