python sort list of objects by attribute based on predefined mapping

Question

How can I sort a list of python objects by attribute based on a defined mapping

E.g.

import random


class Obj(object):
    attr = None

    def __init__(self, attr):
        self.attr = attr

    def __repr__(self):
        return self.attr

attrs = ['GROUPA', 'GROUPB', 'GROUPC']

objects = []

for x in range(0, 10):
    objects.append(Obj(random.choice(attrs)))

objects = sorted(objects, key=lambda o: o.attr)

print '['
for o in objects:
    print o
print ']'

Gives me

[
    GROUPA
    GROUPA
    GROUPA
    GROUPA
    GROUPB
    GROUPB
    GROUPB
    GROUPB
    GROUPC
    GROUPC
]

Which is nice, but I would like to define a mapping and take it into account while sorting, for example:

mapping = ['GROUPB', 'GROUPA', 'GROUPC']

result:

[
    GROUPB
    GROUPB
    GROUPB
    GROUPB
    GROUPA
    GROUPA
    GROUPA
    GROUPA
    GROUPC
    GROUPC
]

and then get the result like this

Running your example does not output `[ AAABBBCCC ]` as you write, but `[ GROUPA GROUPA GROUPA GROUPA GROUPA GROUPB GROUPB GROUPB GROUPC GROUPC ]`. Maybe you should correct the example, or the example output? — logc, Jun 05 '14 at 09:34
What have you tried? Why not implement [comparison operators](https://docs.python.org/2/reference/datamodel.html#object.__lt__) on your class? — jonrsharpe, Jun 05 '14 at 09:36

score 4 · Answer 1 · answered Jun 05 '14 at 09:37

You can define the mapping in a dictionary with the keys as the characters to be mapped and their values integers with the lower value characters having smaller integers as shown below:

mapping = {'B': 1, 'A': 2, 'C': 3}

Then you can use this dictionary in your lambda method as:

sorted(objects, key=lambda o: mapping[o.attr])

linkyndy · Answer 2 · 2014-06-05T09:41:30.970

-1

You can make use of the cmp argument of the sorted builtin. Define a comparing function where you check whether the elements' corresponding position in the mapping list. Something like:

def compare(a, b):
    return cmp(mapping.index(a), mapping.index(b))

Then, you use it like:

objects = sorted(objects, cmp=compare)

Even better, as some suggested in the comments, you can bypass the cmp argument and directly use key:

objects = sorted(objects, key=lambda o: mapping.index(o.attr))

This way, your code will be compatible with Python 3 and it is much easier to understand.

edited Jun 05 '14 at 09:41

answered Jun 05 '14 at 09:36

linkyndy

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`key` is both easier to use and more efficient (it gets called n times, while `cmp` gets called O(n lg n) times): `sorted(objects, key=lambda o: mapping.index(o.attr))`. – Fred Foo Jun 05 '14 at 09:37
2

`cmp` is deprecated, not available in Python 3, and the OP is already using the newer `key` argument, a much better option. – Martijn Pieters Jun 05 '14 at 09:39
Right, I overcomplicated myself :) – linkyndy Jun 05 '14 at 09:39

python sort list of objects by attribute based on predefined mapping

2 Answers2