Python Dictionary setdefault()

The setdefault() method returns the value of a key (if the key is in dictionary). If not, it inserts key with a value to the dictionary.

The syntax of setdefault() is:

dict.setdefault(key[, default_value])

1. setdefault() Parameters

setdefault() takes a maximum of two parameters:

• key – the key to be searched in the dictionary
• default_value (optional) – key with a value default_value is inserted to the dictionary if the key is not in the dictionary.
  If not provided, the default_value will be None.

2. Return Value from setdefault()

setdefault() returns:

• value of the key if it is in the dictionary
• None if the key is not in the dictionary and default_value is not specified
• default_value if key is not in the dictionary and default_value is specified

3. Example 1: How setdefault() works when key is in the dictionary?

person = {'name': 'Phill', 'age': 22}

age = person.setdefault('age')
print('person = ',person)
print('Age = ',age)

Output

person =  {'name': 'Phill', 'age': 22}
Age =  22

4. Example 2: How setdefault() works when key is not in the dictionary?

person = {'name': 'Phill'}

# key is not in the dictionary
salary = person.setdefault('salary')
print('person = ',person)
print('salary = ',salary)

# key is not in the dictionary
# default_value is provided
age = person.setdefault('age', 22)
print('person = ',person)
print('age = ',age)

Output

person =  {'name': 'Phill', 'salary': None}
salary =  None
person =  {'name': 'Phill', 'age': 22, 'salary': None}
age =  22