How to sort objects by multiple keys?

Solution 1:

This article has a nice rundown on various techniques for doing this. If your requirements are simpler than "full bidirectional multikey", take a look. It's clear the accepted answer and the blog post I just referenced influenced each other in some way, though I don't know which order.

In case the link dies here's a very quick synopsis of examples not covered above:

mylist = sorted(mylist, key=itemgetter('name', 'age'))
mylist = sorted(mylist, key=lambda k: (k['name'].lower(), k['age']))
mylist = sorted(mylist, key=lambda k: (k['name'].lower(), -k['age']))

Solution 2:

This answer works for any kind of column in the dictionary -- the negated column need not be a number.

def multikeysort(items, columns):
    from operator import itemgetter
    comparers = [((itemgetter(col[1:].strip()), -1) if col.startswith('-') else
                  (itemgetter(col.strip()), 1)) for col in columns]
    def comparer(left, right):
        for fn, mult in comparers:
            result = cmp(fn(left), fn(right))
            if result:
                return mult * result
        else:
            return 0
    return sorted(items, cmp=comparer)

You can call it like this:

b = [{u'TOT_PTS_Misc': u'Utley, Alex', u'Total_Points': 96.0},
     {u'TOT_PTS_Misc': u'Russo, Brandon', u'Total_Points': 96.0},
     {u'TOT_PTS_Misc': u'Chappell, Justin', u'Total_Points': 96.0},
     {u'TOT_PTS_Misc': u'Foster, Toney', u'Total_Points': 80.0},
     {u'TOT_PTS_Misc': u'Lawson, Roman', u'Total_Points': 80.0},
     {u'TOT_PTS_Misc': u'Lempke, Sam', u'Total_Points': 80.0},
     {u'TOT_PTS_Misc': u'Gnezda, Alex', u'Total_Points': 78.0},
     {u'TOT_PTS_Misc': u'Kirks, Damien', u'Total_Points': 78.0},
     {u'TOT_PTS_Misc': u'Worden, Tom', u'Total_Points': 78.0},
     {u'TOT_PTS_Misc': u'Korecz, Mike', u'Total_Points': 78.0},
     {u'TOT_PTS_Misc': u'Swartz, Brian', u'Total_Points': 66.0},
     {u'TOT_PTS_Misc': u'Burgess, Randy', u'Total_Points': 66.0},
     {u'TOT_PTS_Misc': u'Smugala, Ryan', u'Total_Points': 66.0},
     {u'TOT_PTS_Misc': u'Harmon, Gary', u'Total_Points': 66.0},
     {u'TOT_PTS_Misc': u'Blasinsky, Scott', u'Total_Points': 60.0},
     {u'TOT_PTS_Misc': u'Carter III, Laymon', u'Total_Points': 60.0},
     {u'TOT_PTS_Misc': u'Coleman, Johnathan', u'Total_Points': 60.0},
     {u'TOT_PTS_Misc': u'Venditti, Nick', u'Total_Points': 60.0},
     {u'TOT_PTS_Misc': u'Blackwell, Devon', u'Total_Points': 60.0},
     {u'TOT_PTS_Misc': u'Kovach, Alex', u'Total_Points': 60.0},
     {u'TOT_PTS_Misc': u'Bolden, Antonio', u'Total_Points': 60.0},
     {u'TOT_PTS_Misc': u'Smith, Ryan', u'Total_Points': 60.0}]

a = multikeysort(b, ['-Total_Points', 'TOT_PTS_Misc'])
for item in a:
    print item

Try it with either column negated. You will see the sort order reverse.

Next: change it so it does not use extra class....

2016-01-17

Taking my inspiration from this answer What is the best way to get the first item from an iterable matching a condition?, I shortened the code:

from operator import itemgetter as i

def multikeysort(items, columns):
    comparers = [
        ((i(col[1:].strip()), -1) if col.startswith('-') else (i(col.strip()), 1))
        for col in columns
    ]
    def comparer(left, right):
        comparer_iter = (
            cmp(fn(left), fn(right)) * mult
            for fn, mult in comparers
        )
        return next((result for result in comparer_iter if result), 0)
    return sorted(items, cmp=comparer)

In case you like your code terse.

Later 2016-01-17

This works with python3 (which eliminated the cmp argument to sort):

from operator import itemgetter as i
from functools import cmp_to_key

def cmp(x, y):
    """
    Replacement for built-in function cmp that was removed in Python 3

    Compare the two objects x and y and return an integer according to
    the outcome. The return value is negative if x < y, zero if x == y
    and strictly positive if x > y.

    https://portingguide.readthedocs.io/en/latest/comparisons.html#the-cmp-function
    """

    return (x > y) - (x < y)

def multikeysort(items, columns):
    comparers = [
        ((i(col[1:].strip()), -1) if col.startswith('-') else (i(col.strip()), 1))
        for col in columns
    ]
    def comparer(left, right):
        comparer_iter = (
            cmp(fn(left), fn(right)) * mult
            for fn, mult in comparers
        )
        return next((result for result in comparer_iter if result), 0)
    return sorted(items, key=cmp_to_key(comparer))

Inspired by this answer How should I do custom sort in Python 3?