Python Dictionary setdefault() Method

value = my_dict.setdefault(key, default) returns the value for an existing key. If the key is missing, it inserts the key with the supplied default value and returns that value.

See the complete Dictionary Methods table for the other dictionary operations in this cluster.

my_dict={'a':'One','b':'Two','c':'Three'}
x=my_dict.setdefault('b')
print(x)
print(my_dict)

Two
{'a': 'One', 'b': 'Two', 'c': 'Three'}

If input key is not available then key is added.

my_dict={'a':'One','b':'Two','c':'Three'}
x=my_dict.setdefault('d')
print(x)
print(my_dict)

None
{'a': 'One', 'b': 'Two', 'c': 'Three', 'd': None}

Adding new key with value

my_dict={'a':'One','b':'Two','c':'Three'}
x=my_dict.setdefault('d','Four')
print(x)
print(my_dict)

Four
{'a': 'One', 'b': 'Two', 'c': 'Three', 'd': 'Four'}

Example 1: Setting Default Value with a Different Data Type

my_dict = {'a': 1, 'b': 2}
x = my_dict.setdefault('c', [3, 4])
print(x)  # Output: [3, 4]
print(my_dict)  # Output: {'a': 1, 'b': 2, 'c': [3, 4]}

Example 2: Using setdefault() with Nested Dictionaries

nested_dict = {'person': {'name': 'Alice'}}
nested_dict.setdefault('person', {}).setdefault('age', 25)
print(nested_dict)  # Output: {'person': {'name': 'Alice', 'age': 25}}

Example 3: Counting Occurrences in a List with setdefault()

my_list = ['apple', 'banana', 'apple', 'orange']
count_dict = {}
for item in my_list:
    count_dict.setdefault(item, 0)
    count_dict[item] += 1
print(count_dict)  # Output: {'apple': 2, 'banana': 1, 'orange': 1}

Example 4: Handling Missing Keys with Default Values

data = {'name': 'Alice'}
age = data.setdefault('age', 30)
print(age)  # Output: 30
print(data)  # Output: {'name': 'Alice', 'age': 30}

Example 5: Appending to a List in a Dictionary Using setdefault()

my_dict = {}
my_dict.setdefault('fruits', []).append('apple')
my_dict.setdefault('fruits', []).append('banana')
print(my_dict)  # Output: {'fruits': ['apple', 'banana']}

Example 6: Grouping Strings by Length Using setdefault()

words = ['cat', 'dog', 'elephant', 'bird']
length_dict = {}
for word in words:
    length_dict.setdefault(len(word), []).append(word)
print(length_dict)  # Output: {3: ['cat', 'dog'], 8: ['elephant'], 4: ['bird']}

Subhendu Mohapatra

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