In this tutorial, I will explain how to sort a list of objects by attribute in Python. As a developer working on a project, I came across a scenario where I needed to sort a list of objects by attribute. Then, I explored more about this topic. After researching and testing various methods, I found a few effective methods to accomplish this task.
Sort a List of Objects by Attribute in Python
Let us learn how to sort a list of objects by attribute in Python:
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1. Use a Lambda Function
A lambda function in Python is an anonymous function that can be used to define custom sorting logic. It is flexible and allows you to sort by multiple attributes.
Example:
Sort a list of US states by their name and population.
# List of tuples: (State Name, Population)
states = [
("California", 39512223),
("Texas", 28995881),
("Florida", 21477737),
("New York", 19453561),
("Pennsylvania", 12801989),
]
# Sort by state name (alphabetically) and then by population (ascending)
sorted_states = sorted(states, key=lambda x: (x[0], x[1]))
print("Sorted by name and population:")
for state in sorted_states:
print(state)Output:
Sorted by name and population:
('California', 39512223)
('Florida', 21477737)
('New York', 19453561)
('Pennsylvania', 12801989)
('Texas', 28995881)You can look at the output in the screenshot below.
The lambda x: (x[0], x[1]) function sorts the list first by the state name (x[0]) and then by population (x[1]). The sorted() function returns a new sorted list.
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2. Use itemgetter
The itemgetter function from the operator module is used to retrieve specific items from an iterable (e.g., lists, tuples). It is efficient and often faster than a lambda function for simple cases.
Example:
Sort a list of US states by their population and then by name.
from operator import itemgetter
# List of tuples: (State Name, Population)
states = [
("California", 39512223),
("Texas", 28995881),
("Florida", 21477737),
("New York", 19453561),
("Pennsylvania", 12801989),
]
# Sort by population (ascending) and then by state name (alphabetically)
sorted_states = sorted(states, key=itemgetter(1, 0))
print("Sorted by population and name:")
for state in sorted_states:
print(state)Output:
Sorted by population and name:
('Pennsylvania', 12801989)
('New York', 19453561)
('Florida', 21477737)
('Texas', 28995881)
('California', 39512223)You can look at the output in the screenshot below.
itemgetter(1, 0) retrieves the population (1) and then the state name (0) for sorting. The sorted() function sorts the list based on the specified keys.
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3. Use attrgetter
The attrgetter function from the operator module is used to retrieve attributes from objects. It is useful when working with lists of objects instead of lists of tuples or lists.
Example:
Sort a list of US state objects by their name and population.
from operator import attrgetter
# Define a State class
class State:
def __init__(self, name, population):
self.name = name
self.population = population
def __repr__(self):
return f"{self.name} (Population: {self.population})"
# List of State objects
states = [
State("California", 39512223),
State("Texas", 28995881),
State("Florida", 21477737),
State("New York", 19453561),
State("Pennsylvania", 12801989),
]
# Sort by state name (alphabetically) and then by population (ascending)
sorted_states = sorted(states, key=attrgetter('name', 'population'))
print("Sorted by name and population:")
for state in sorted_states:
print(state)Output:
Sorted by name and population:
California (Population: 39512223)
Florida (Population: 21477737)
New York (Population: 19453561)
Pennsylvania (Population: 12801989)
Texas (Population: 28995881)You can look at the output in the screenshot below.
attrgetter('name', 'population') retrieves the name and population attributes from each State object for sorting. The sorted() function sorts the list of objects based on the specified attributes.
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Sort by Multiple Attributes
Let us see how to demonstrate sorting a list of employees by their salary in ascending order:
# Define the Employee class and sort employees by salary in one step
class Employee:
def __init__(self, name, age, salary):
self.name = name
self.age = age
self.salary = salary
def __repr__(self):
return f"{self.name} (Age: {self.age}, Salary: ${self.salary})"
# Create a list of Employee objects and sort by salary
employees = sorted([
Employee("Alice Johnson", 28, 90000),
Employee("Bob Smith", 34, 75000),
Employee("Charlie Brown", 22, 60000),
Employee("David Wilson", 40, 120000),
Employee("Eva Green", 29, 80000),
], key=lambda x: x.salary)
# Print sorted employees
for employee in employees:
print(employee)Output:
Charlie Brown (Age: 22, Salary: $60000)
Bob Smith (Age: 34, Salary: $75000)
Eva Green (Age: 29, Salary: $80000)
Alice Johnson (Age: 28, Salary: $90000)
David Wilson (Age: 40, Salary: $120000)The Employee class is defined with name, age, and salary attributes. A list of Employee objects is created. The sorted() function sorts the list by the salary attribute using a lambda function (key=lambda x: x.salary). The sorted list is printed, showing employees ordered by salary in ascending order.
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Conclusion
In this article, I have explained how to sort a list of objects by attribute in Python. I discussed three methods such as using lambda function, using itemgetter, and using attrgetter. I also covered how to sort by multiple attributes.
You may also read:
- How to Add Elements to an Empty List in Python?
- How to Check if an Element is Not in a List in Python?
- How to Clear a List in Python?
I am Bijay Kumar, a Microsoft MVP in SharePoint. Apart from SharePoint, I started working on Python, Machine learning, and artificial intelligence for the last 5 years. During this time I got expertise in various Python libraries also like Tkinter, Pandas, NumPy, Turtle, Django, Matplotlib, Tensorflow, Scipy, Scikit-Learn, etc… for various clients in the United States, Canada, the United Kingdom, Australia, New Zealand, etc. Check out my profile.
