What is the purpose of the defaultdict in Python?

Python Data Structures - Interview Questions and Answers

Purpose of `defaultdict` in Python

defaultdict is a subclass of Python’s built-in dict that automatically provides a default value for missing keys, preventing KeyError. It’s part of the collections module.

1. Why Use `defaultdict`?

Avoids KeyError when accessing missing keys.
Automatically assigns a default value to new keys.
Simplifies code, especially when handling nested structures like lists or sets.

2. Basic Usage of `defaultdict`

Without `defaultdict` (Using Normal `dict`)

my_dict = {}
my_dict["a"].append(1)  # ? KeyError: 'a' (Key does not exist)

With `defaultdict` (Prevents `KeyError`)

from collections import defaultdict

# Creates a defaultdict with list as the default factory
my_dict = defaultdict(list)

# No KeyError, automatically initializes 'a' as an empty list
my_dict["a"].append(1)

print(my_dict)  # Output: {'a': [1]}

* No KeyError! defaultdict automatically creates an empty list.

3. Default Factories in `defaultdict`

You can pass a factory function to specify default values:

Default Factory	Example	Default Value
`list`	`defaultdict(list)`	`[]` (Empty list)
`int`	`defaultdict(int)`	`0`
`set`	`defaultdict(set)`	`set()` (Empty set)
`str`	`defaultdict(str)`	`''` (Empty string)
`lambda: custom_value`	`defaultdict(lambda: 100)`	`100`

Example: Using `int` to Count Occurrences

word_count = defaultdict(int)
words = ["apple", "banana", "apple", "orange", "banana", "banana"]

for word in words:
    word_count[word] += 1  # No need to check if key exists!

print(word_count)  # Output: {'apple': 2, 'banana': 3, 'orange': 1}

* No need to initialize keys manually!

4. Nested `defaultdict` (Creating Auto-Nested Dictionaries)

defaultdict can be used to create nested dictionaries without manually initializing sub-dictionaries.

Example : Creating a Nested Dictionary

nested_dict = defaultdict(lambda: defaultdict(int))

nested_dict["A"]["Math"] = 90
nested_dict["A"]["Science"] = 85

print(nested_dict)  
# Output: {'A': {'Math': 90, 'Science': 85}}

* Automatically creates sub-dictionaries when accessing missing keys.

5. When to Use `defaultdict` vs. `dict`?

Feature	`dict`	`defaultdict`
Handles missing keys	? No (`KeyError`)	? Yes (Auto-initialized)
Requires manual initialization	? Yes	? No
Best for	Simple lookups	Grouping, counting, nesting

When Should You Use `defaultdict`?

* When grouping elements (defaultdict(list))
* When counting occurrences (defaultdict(int))
* When creating nested dictionaries (defaultdict(lambda: defaultdict(int))

Purpose of defaultdict in Python

1. Why Use defaultdict?

2. Basic Usage of defaultdict

Without defaultdict (Using Normal dict)

With defaultdict (Prevents KeyError)

3. Default Factories in defaultdict

Example: Using int to Count Occurrences

4. Nested defaultdict (Creating Auto-Nested Dictionaries)