Defaultdict in Python

In Python, defaultdict is a subclass of the built-in dict class from the collections module. It is used to provide a default value for a nonexistent key in the dictionary, eliminating the need for checking if the key exists before using it.

Key Features of defaultdict:

• When we access a key that doesn’t exist in the dictionary, defaultdict automatically creates it and assigns it a default value based on a function we provide.
• We need to specify the default value type by passing a function (like int, list or set) when initializing the defaultdict.

Example:

from collections import defaultdict

d = defaultdict(list)

d['fruits'].append('apple')
d['vegetables'].append('carrot')
print(d)

print(d['juices'])

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

Explanation: This code creates a defaultdict with a default value of an empty list. It adds elements to the ‘fruits’ and ‘vegetables’ keys. When trying to access the ‘juices’ key, no KeyError is raised, and an empty list is returned since it doesn’t exist in the dictionary.

Syntax of DefaultDict in Python

defaultdict(default_factory)

Parameters:

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

Return Value: It returns a dictionary-like object that automatically provides a default value for missing keys, based on the specified callable, instead of raising a KeyError.

How Does defaultdict Work?

When a defaultdict is created, you specify a factory function that will provide the default value for new keys. This factory function could be int, list, str, or any other callable object. For example:

• Using int: If you use int as the factory function, the default value will be 0 (since int() returns 0).
• Using list: If you use list as 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 returning the default value for the dictionary defined. If this argument is absent then the dictionary raises a KeyError.

from collections import defaultdict

    
# Defining the dict and passing lambda as 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

Use Cases for defaultdict

1. Using List as Default Factory

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

from collections import defaultdict

d = defaultdict(list)

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

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

Explanation: This example demonstrates the use of list as the default factory. A defaultdict is created with list, which means any missing key will automatically have an empty list as its value. The loop appends the value of i to the list of the corresponding key.

2. Using int Default Factory

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

from collections import defaultdict
 
d = defaultdict(int)
 
a = [1, 2, 3, 4, 2, 4, 1, 2]
 

for i in a:

    d[i] += 1
     
print(d)

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

Explanation: This example uses int as the default factory. int() returns 0, so missing keys will have a default value of 0. The loop counts the occurrences of each number in the list a and updates the dictionary accordingly.

3. Using str Default Factory

When the str class is passed as the default_factory argument.

from collections import defaultdict

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

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

Explanation: This example uses str as the default factory. str() returns an empty string, so missing keys will have an empty string as their default value. A value (‘Hello‘) is explicitly set for the key ‘greeting‘.

Python defaultdict Type for Handling Missing Keys

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

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

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

print(d.__missing__('a'))
print(d.__missing__('d'))

Not Present
Not Present

Explanation:

• The defaultdict is initialized with a lambda function that returns “Not Present” for missing keys.
• When the key ‘a‘ is accessed, its value is returned (1), and __missing__ is not called.
• When the key ‘d‘ is accessed (which does not exist), __missing__ is triggered, and the default value “Not Present” is returned.