Python Dictionary setdefault() Method

Rate this post

Related Tutorial:

Syntax

dict.setdefault(key, default_value)

Parameters

  • The dict.setdefault() method takes two parameters, the key and the default_value of the key. The default value is optional.

Return Value

  • Returns the value associated with the passed key.
  • If the key is not in the dictionary, returns the default value.
  • If the key is not in the dictionary and no default value is given, returns None.

Example setdefault()

Example showing dict.setdefault() method operating on a dictionary:

employee = {'id': 1, 'name': 'sarah', 'age': 21}
employee_name = employee.setdefault('name')
 
print('employees full information: ', employee)
print('employees name: ', employee_name)

Output:

employees full information:  {'id': 1, 'name': 'sarah', 'age': 21}
employees name:  sarah

This example shows the dict.setdefault() method is applied to the employee dictionaries’ second key β€˜name’, which then sets the keys associated value β€˜sarah’ as the default value, as shown in the printed results.

Example Key Does Not Exist

Let’s explore an example where the key is not in a dictionary:

employees = {'id_1': 'tammy', 'id_2': 'bob'}
id_4 = employees.setdefault('id_4')
 
print('employees: ', employees)
print('employee name of 4th id: ', id_4)

Output:

employees:  {'id_1': 'tammy', 'id_2': 'bob', 'id_4': None}
employee name of 4th id:  None

This example shows that when the dict.setdefault() method is applied to a key that’s not in the employees dictionary, then the None value gets returned.

Example Setting Default Value

Example of setting a custom default value for a key that’s not in a Python dictionary using the dict.setdefault() method:

items = {'item_1': 2, 'item_2': 5, 'item_3': 8}
item_4 = items.setdefault('item_4', 6)
 
print('dictionary of items: ', items)
print('value of 4th item not found in dictionary: ', item_4)

Output:

dictionary of items:  {'item_1': 2, 'item_2': 5, 'item_3': 8, 'item_4': 6}
value of 4th item not found in dictionary:  6

The results thow how a default_value can be set for a key not found in a dictionary using the dict.setdefault() method.

setdefault() vs defaultdict()

An example showing how the dict.setdefault() method differs to collections.defaultdict():

import collections
 
# setdefault:
fruits = {'peaches': 3, 'apples': 6, 'apricots': 4}
apples = fruits.setdefault('apples', 3)
print('quantity of apples in the fruit basket: ', apples)
 
 
def default_value():
   return 'item is not present in this dictionary'
 
# defaultdict:
items = collections.defaultdict(default_value)
 
items['pens'] = 3
items['notebooks'] = 6
 
print('quantity of pens: ', items['pens'])
print('quantity of notebooks: ', items['notebooks'])
print('quantity of pencils: ', items['pencils'])

Output:

quantity of apples in the fruit basket:  6
quantity of pens:  3
quantity of notebooks:  6
quantity of pencils:  item is not present in this dictionary

The dict.setdefault() method sets a dictionary key to a default value if the key does not exist in the dictionary.

The collections.defaultdict() method differs from the dict.setdefault() method in that it automatically populates every key in the dictionary with a default value.