Python Built-in Methods – Python Dictionary setdefault() Method

Python Dictionary setdefault() Method

Returns the value for key if exists, else inserts it

Usage

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

Syntax

dictionary.setdefault(key,default)

Parameter Condition Description
key Required Any key you want to return value for
default Optional A value to insert if the specified key is not found.
Default value is None.

Basic Example

setdefault() method is generally used to insert a key with a default value.

# Insert a key 'job' with default value 'Dev'
D = {'name': 'Bob', 'age': 25}
v = D.setdefault('job', 'Dev')
print(D)
# Prints {'job': 'Dev', 'age': 25, 'name': 'Bob'}
print(v)
# Prints Dev

setdefault() Method Scenarios

The method’s output depends on input parameters. Here are three scenarios for different input parameters.

Key Present

If key is in the dictionary, the method returns the value for key (no matter what you pass in as default)

# without default specified
D = {'name': 'Bob', 'age': 25}
v = D.setdefault('name')
print(v)
# Prints Bob

# with default specified
D = {'name': 'Bob', 'age': 25}
v = D.setdefault('name', 'Max')
print(v)
# Prints Bob

Key Absent, Default Specified

If key is not in the dictionary, the method inserts key with a value of default and returns default.

D = {'name': 'Bob', 'age': 25}
v = D.setdefault('job', 'Dev')
print(D)
# Prints {'job': 'Dev', 'age': 25, 'name': 'Bob'}
print(v)
# Prints Dev

Key Absent, Default Not Specified

If key is not in the dictionary and default is not specified, the method inserts key with a value None and returns None.

D = {'name': 'Bob', 'age': 25}
v = D.setdefault('job')
print(D)
# Prints {'job': None, 'age': 25, 'name': 'Bob'}
print(v)
# Prints None

 

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