1. Introduction

Let's look at how to sort a list of dictionaries in Python by a specific key value (e.g., age).

student_list = [
  {'name': 'Homer', 'age': 39},
  {'name': 'Homer1', 'age': 20},
  {'name': 'Homer2', 'age': 5},
  {'name': 'Bart', 'age': 10}
]

For sorting lists, Python provides the list.sort() and sorted() functions by default.

• list.sort()
  • sorts in place by modifying the list directly
  • can only be used on the list data structure

def test_simple_sort(self):
  a_list = [3, 2, 1, 7]
  a_list.sort()
  self.assertEqual(a_list, [1, 2, 3, 7])

• sorted()
  • this function returns a new sorted list
  • can also be used on iterable data structures

def test_simple_sorted(self):
  a_list = [3, 2, 1, 7]
  new_list = sorted(a_list)
  self.assertEqual(new_list, [1, 2, 3, 7])

2. Sorting a List of Dictionaries by a Specific Key

You can specify a key function as an argument to the list.sort() and sorted() functions. The return value of this key function is compared to perform the sort.

2.1 Using a Lambda as the Key Function

Specify a lambda function that returns the age value from the dictionary as the key function to sort by age.

def sort_list_by_age_using_lambda(student_list):
    return sorted(student_list, key=lambda k: k['age'])

When you check the sorted result, you can confirm that it is sorted correctly by age.

def test_sort_list_by_age_using_lambda(self):
  result = sort.sort_list_by_age_using_lambda(self.student_list)
  for i in range(len(result) - 1):
    print(i, result[i].get('age'))
    self.assertLess(result[i].get('age'), result[i + 1].get('age'))

2.2 Using itemgetter as the Key Function

The operator module provides several functions to easily access keys or attribute values in data structures. You can specify the key function using the itemgetter() method from the operator module.

from _operator import itemgetter

def sort_list_by_age_using_itemgetter(student_list):
  return sorted(student_list, key=itemgetter('age'))

It is sorted by age in the same way.

def test_sort_list_by_age_using_itemgetter(self):
  result = sort.sort_list_by_age_using_itemgetter(self.student_list)
  for i in range(len(result) - 1):
    print(i, result[i].get('age'))
    self.assertLess(result[i].get('age'), result[i + 1].get('age'))

2.3 Sorting by Multiple Keys

You can also sort by multiple key values (e.g., age -> name). Let's sort by name when ages are the same in the dictionary.

When using a lambda function and the itemgetter() function, you can specify it as follows.

def sort_list_by_two_keys_using_lambda(student_list):
  return sorted(student_list, key=lambda k: (k['age'], k['name']))

def sort_list_by_two_keys_using_itemgetter(student_list):
  return sorted(student_list, key=itemgetter('age', 'name'))

It sorts by age, and for entries with the same age, it sorts by name.

def test_sort_list_by_two_keys_using_itemgetter(self):
  student_list = [
    {'name': 'Homer', 'age': 39},
    {'name': 'Homer2', 'age': 5},
    {'name': 'Homer3', 'age': 5},
    {'name': 'Bart', 'age': 10},
  ]
  result = sort.sort_list_by_two_keys_using_itemgetter(student_list)
  for i in range(len(result) - 1):
    if result[i].get('age') == result[i + 1].get('age'):
      self.assertLess(result[i].get('name'), result[i + 1].get('name'))
     else:
       self.assertLess(result[i].get('age'), result[i + 1].get('age'))

3. Summary

Since you can specify a key function in the sorted() function, you can easily sort by various key values. In addition to a single key, you can sort by multiple key values.

4. References

• https://stackoverflow.com/questions/72899/how-do-i-sort-a-list-of-dictionaries-by-a-value-of-the-dictionary
• https://www.geeksforgeeks.org/ways-sort-list-dictionaries-values-python-using-lambda-function/
• https://www.geeksforgeeks.org/ways-sort-list-dictionaries-values-python-using-itemgetter/
• https://wayhome25.github.io/python/2017/03/07/key-function/