How to Sort List of Dictionaries by Key in Python

To sort a list of dictionaries by a key in Python, use the sorted() function with a key argument that extracts the dictionary value by that key, typically using a lambda function. For example, sorted(list_of_dicts, key=lambda x: x['key_name']) sorts the list by the values of 'key_name'.

Syntax

The basic syntax to sort a list of dictionaries by a key is:

sorted(list_of_dicts, key=lambda x: x['key_name'])

• list_of_dicts: Your list containing dictionaries.
• key=: A function that tells Python which dictionary value to use for sorting.
• lambda x: x['key_name']: A small function that gets the value of 'key_name' from each dictionary.

python

sorted(list_of_dicts, key=lambda x: x['key_name'])

Example

This example shows how to sort a list of dictionaries by the key 'age' in ascending order.

python

people = [
    {'name': 'Alice', 'age': 30},
    {'name': 'Bob', 'age': 25},
    {'name': 'Charlie', 'age': 35}
]
sorted_people = sorted(people, key=lambda x: x['age'])
print(sorted_people)

Output

[{'name': 'Bob', 'age': 25}, {'name': 'Alice', 'age': 30}, {'name': 'Charlie', 'age': 35}]

Common Pitfalls

Common mistakes include:

• Using list.sort() without a key function, which won't sort dictionaries by a specific key.
• Trying to sort by a key that does not exist in all dictionaries, causing errors.
• Forgetting that sorted() returns a new list and does not change the original list.

Here is an example of a wrong and right way:

python

# Wrong: This will cause an error if 'age' key is missing
people = [{'name': 'Alice'}, {'name': 'Bob', 'age': 25}]
# sorted_people = sorted(people, key=lambda x: x['age'])  # KeyError

# Right: Use dict.get() with a default value to avoid errors
sorted_people = sorted(people, key=lambda x: x.get('age', 0))
print(sorted_people)

Output

[{'name': 'Alice'}, {'name': 'Bob', 'age': 25}]

Quick Reference

Tips for sorting lists of dictionaries:

• Use sorted() for a new sorted list or list.sort() to sort in place.
• Use key=lambda x: x['key_name'] to specify the sorting key.
• Add reverse=True to sort in descending order.
• Handle missing keys safely with dict.get().

Key Takeaways

Use sorted(list_of_dicts, key=lambda x: x['key_name']) to sort by a dictionary key.

sorted() returns a new sorted list; list.sort() sorts the list in place.

Handle missing keys safely using dict.get() with a default value.

Add reverse=True to sort in descending order.

Always ensure the key exists or provide a fallback to avoid errors.