sorting of list of list by considering two or three index in python

Question

list = [["hasan",6,"bad","chennai"], ["vishnu",7,"good","chennai"], ["tabraiz",8,"good","bangalore"], ["shaik",5,"excellent","chennai"], ["mani",6,"avarage","kerala"], ["cilvin",9,"excellent","chennai"]]

given priority as

{'excellent':4, 'good':3, 'average':2, 'bad':1}    

list.sort(key=lambda x: (x[3], -x[2], -x[1]))
list.sort(key=lambda x: (x[2], x[0], -x[1]))
list.sort(key=lambda x: (x[2], -x[3], x[0]))

These all are working.

but it says

TypeError: bad operand type for unary -: 'str'

when trying to place negative sign in the first position.

ie

list.sort(key=lambda x: (-x[3], x[2], x[1]))

why????

Unary minus operator works properly, in sorting, with numeric data only. — thefourtheye
– thefourtheye, Commented Jan 17, 2014 at 9:28
Well, is the first element of each sub-list a string? Because that would cause exactly this problem. — senshin
– senshin, Commented Jan 17, 2014 at 9:29
These all are working. I do doubt that. Please double check it. To be specific, only list.sort(key=lambda x: (x[2], x[0], -x[1])) shall work. Also don't override built-in identifiers like list. — starrify
– starrify, Commented Jan 17, 2014 at 9:39
What are you trying to achieve? Do you want to order first by x[3] in decreasing order, then by x[2] in increasing order and then by x[1] in increasing order? — Bakuriu
– Bakuriu, Commented Jan 17, 2014 at 9:41

eskaev · Accepted Answer · 2014-01-17 11:27:11Z

The problem with your method is that x[3] is a string, so it does not make sense to do -x[3].

I am not sure there isn't a simpler solution, but you can use cmp_to_key, and define your own reversed key function:

from functools import cmp_to_key

l = [['hasan', 6, 'bad', 'chennai'],
     ['vishnu', 7, 'good', 'chennai'],
     ['tabraiz', 8, 'good', 'bangalore'],
     ['shaik', 5, 'excellent', 'chennai'],
     ['mani', 6, 'avarage', 'kerala'],
     ['cilvin', 9, 'excellent', 'chennai']]

rev_key = cmp_to_key(lambda x,y: 1 if x<y else -1 if x>y else 0)
l.sort(key=lambda x: (rev_key(x[3]), x[2], x[1]))
print(l)

Result:

[['mani', 6, 'avarage', 'kerala'],
 ['hasan', 6, 'bad', 'chennai'],
 ['shaik', 5, 'excellent', 'chennai'],
 ['cilvin', 9, 'excellent', 'chennai'],
 ['vishnu', 7, 'good', 'chennai'],
 ['tabraiz', 8, 'good', 'bangalore']]

Note that if you're using Python 2, you can just write:

rev_key = cmp_to_key(lambda x,y: cmp(y,x))

If you didn't want to sort according to several keys, the right way to go would be to use the reverse argument:

>>> sorted(l, key=lambda x: x[3], reverse=True)
[['mani', 6, 'avarage', 'kerala'],
 ['hasan', 6, 'bad', 'chennai'],
 ['vishnu', 7, 'good', 'chennai'],
 ['shaik', 5, 'excellent', 'chennai'],
 ['cilvin', 9, 'excellent', 'chennai'],
 ['tabraiz', 8, 'good', 'bangalore']]

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