How to Sort a List of Objects by an Object Attribute in Python

By Birtchum Thompson | April 12, 2020

In some cases it is useful to sort a list of objects alphabetically or by an attribute like an ID number. The Python operator module allows us to easily achieve this.

First, import the operator module and create the Bird class. The Bird class will accept 2 arguments: kind and wings. Wings is set to True by default meaning we will not have to pass this value when instantiating Bird objects.

import operator

class Bird:
    def __init__(self, kind, wings=True):
        self.kind = kind
        self.wings = wings

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Next, we create our sort function and control-flow logic. The sort_alphabetically function will receive a list of Bird objects and a reverse directive, which is set to False by default. The Bird objects are then sorted alphabetically by kind. The control-flow logic contains a list of bird kinds and a list of Bird objects created from the bird kinds list.

import operator

class Bird:
    def __init__(self, kind, wings=True):
        self.kind = kind
        self.wings = wings

def sort_alphabetically(birds, reverse=False):
    return sorted(
        birds, 
        key=operator.attrgetter('kind'),
        reverse=reverse)

if __name__ == '__main__':
    kinds = ['robbin', 'canary', 'seagull', 'peacock']
    birds = [Bird(kind) for kind in kinds]

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Finally, we can start testing the program. First, we will print out the list of bird objects before sorting as a reference point by adding the following for-loop to our program.

import operator

class Bird:
    def __init__(self, kind, wings=True):
        self.kind = kind
        self.wings = wings

def sort_alphabetically(birds, reverse=False):
    return sorted(
        birds, 
        key=operator.attrgetter('kind'),
        reverse=reverse)

if __name__ == '__main__':
    kinds = ['robbin', 'canary', 'seagull', 'peacock']
    birds = [Bird(kind) for kind in kinds]

    # Not Sorted
    for bird in birds:
        print(bird.kind)

Python

Output:

robbin
canary
seagull
peacock

Then we sort the Bird objects alphabetically and print their kinds.

import operator

class Bird:
    def __init__(self, kind, wings=True):
        self.kind = kind
        self.wings = wings

def sort_alphabetically(birds, reverse=False):
    return sorted(
        birds, 
        key=operator.attrgetter('kind'),
        reverse=reverse)

if __name__ == '__main__':
    kinds = ['robbin', 'canary', 'seagull', 'peacock']
    birds = [Bird(kind) for kind in kinds]

    # Not Sorted
    '''for bird in birds:
        print(bird.kind)'''

    # Sorted
    sorted_birds = sort_alphabetically(birds)
    for bird in sorted_birds:
        print(bird.kind)

Python

Output:

canary
peacock
robbin
seagull

Lastly, we sort in reverse-alphabetical order and print the Bird object kinds.

import operator

class Bird:
    def __init__(self, kind, wings=True):
        self.kind = kind
        self.wings = wings

def sort_alphabetically(birds, reverse=False):
    return sorted(
        birds, 
        key=operator.attrgetter('kind'),
        reverse=reverse)

if __name__ == '__main__':
    kinds = ['robbin', 'canary', 'seagull', 'peacock']
    birds = [Bird(kind) for kind in kinds]

    # Not Sorted
    '''for bird in birds:
        print(bird.kind)'''

    # Sorted
    '''sorted_birds = sort_alphabetically(birds)
    for bird in sorted_birds:
        print(bird.kind)'''

    # Sorted Reverse
    sorted_birds = sort_alphabetically(birds, True)
    for bird in sorted_birds:
        print(bird.kind)

Python

Output: