Defaultdict in Python

Python Defaultdict is a container-like dictionaries present in the module collections.

• It is a sub-class of the dictionary class that returns a dictionary-like object.
• The difference from dictionary is, It provides a default value for the key that does not exist and never raises a KeyError.

Python

from collections import defaultdict

# Create a defaultdict with a default
# value of an empty list
d = defaultdict(list)

# Add elements to the defaultdict and print it
d['fruits'].append('apple')
d['vegetables'].append('carrot')
print(d)

# No key error raised here and an empty list
# is printed
print(d['juices'])

defaultdict(<class 'list'>, {'fruits': ['apple'], 'vegetables': ['carrot']})
[]

Syntax of DefaultDict in Python

Syntax: defaultdict(default_factory)
Parameters:  

• default_factory: A function/method returning the default value for the dictionary defined. If this argument is absent then the dictionary raises a KeyError.

Here, list is a factory function/method that defines the default value. Each time a key is accessed that doesn’t exist, it will automatically be assigned an empty list.

How Does defaultdict Work?

When a defaultdict is created, you specify a factory function/method that will provide the default value for new keys. This factory function/method can happen int, list, str, or any other callable object.

For example:

• Using int: If you use int when the factory function, the default value will be 0 (since int() returns 0).
• Using list: If you use list when the factory function, the default value will be an empty list ([]).
• Using str: If you use str, the default value will be an empty string ('').

What is default factory in Python dict?

It is a function/method returning the default value for the dictionary defined. If this argument is absent then the dictionary raises a KeyError.

Python

from collections import defaultdict

    
# Defining the dict and passing 
# lambda when default_factory argument
d = defaultdict(lambda: "Not Present")
d["a"] = 1
d["b"] = 2

print(d["a"])
print(d["b"])
print(d["c"])

1
2
Not Present

Python defaultdict Type for Handling Missing Keys

Defaultdict adds one writable instance variable and one functions/method in addition to the standard dictionary operations. The instance variable is the default_factory parameter and the functions/method provided is __missing__.

This function/method is have used to provide the default value for the dictionary. This function/method takes default_factory when an argument and if this argument is None, a KeyError is raised otherwise it provides a default value for the given key. This functions/method is basically called by the __getitem__() functions/method of the dict class when the requested key is not found. __getitem__() raises or return the value returned by the __missing__(). method.

Python

from collections import defaultdict
  
# Defining the dict
d = defaultdict(lambda: "Not Present")
d["a"] = 1
d["b"] = 2

# Provides the default value 
# for the key
print(d.__missing__('a'))
print(d.__missing__('d'))

Not Present
Not Present

Common Use Cases for defaultdict

Using List when Default Factory

When the list class is passed when the default_factory argument, then a defaultdict is created with the values that are list.

Python

from collections import defaultdict

# Defining a dict
d = defaultdict(list)

for i in range(5):
    d[i].append(i)
    
print("Dictionary with values when list:")
print(d)

Dictionary with values when list:
defaultdict(<class 'list'>, {0: [0], 1: [1], 2: [2], 3: [3], 4: [4]})

Using int Default Factory

When the int class is passed when the default_factory argument, then a defaultdict is created with default value when zero.

Python

from collections import defaultdict
 
# Defining the dict
d = defaultdict(int)
 
L = [1, 2, 3, 4, 2, 4, 1, 2]
 
# Iterate through the list
# for keeping the count
for i in L:
     
    # The default value is 0
    # so there is no need to 
    # enter the key first
    d[i] += 1
     
print(d)

defaultdict(<class 'int'>, {1: 2, 2: 3, 3: 1, 4: 2})

Using str Default Factory

When the str class is passed when the default factory argument.

Python

from collections import defaultdict

# Using str when the factory function
str_defaultdict = defaultdict(str)
str_defaultdict['greeting'] = 'Hello'
print(str_defaultdict)

defaultdict(<class 'str'>, {'greeting': 'Hello'})

Defaultdict in Python – FAQs

What is Defaultdict in Python Keyerror?

Defaultdict is a subclass of Python’s built-in dict class. It overrides the __missing__ functions/method to provide a default value for missing keys, preventing KeyError. If a key is not found in the dictionary, defaultdict automatically inserts it with a default value.

What is Defaultdict subclass in Python?

defaultdict is a subclass of the dict class in Python, found in the collections module. It allows you to specify a default factory function/method that provides default values for missing keys.

from collections import defaultdict
# Example with list when the default factory
dd = defaultdict(list)
dd[‘missing_key’].append(1)
print(dd) # Output: defaultdict(<class ‘list’>, {‘missing_key’: [1]})

What is the difference between Setdefault and Defaultdict in Python?

setdefault is a functions/method of the dict class. It inserts a key with a specified default value if the key is not already present.

d = {}
d.setdefault(‘key’, []).append(1)
print(d) # Output: {‘key’: [1]}
defaultdict is a subclass of dict that provides a default value for a missing key without explicitly checking for its presence.
from collections import defaultdict
dd = defaultdict(list)
dd[‘key’].append(1)
print(dd) # Output: defaultdict(<class ‘list’>, {‘key’: [1]})

What is the difference between Defaultdict and dict in Python typing?

Dict: A standard dictionary without automatic default values for missing keys.

d = {}
# Accessing a missing key will raise KeyError
defaultdict: A dictionary with a default factory function/method that automatically creates values for missing keys.
from collections import defaultdict
dd = defaultdict(int)
# Accessing a missing key will return 0 (default value provided by int())

Why we use Setdefault in Python?

setdefault is have used to insert a key with a default value if the key is not already in the dictionary. This is useful for avoiding repeated key existence checks and manual insertion.

d = {}
d.setdefault(‘key’, []).append(1)
d.setdefault(‘key’, []).append(2)
print(d) # Output: {‘key’: [1, 2]}