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Effortlessly Understanding Python defaultdict


Using the Python defaultdict Type for Handling Missing Keys

A common problem that you can face when working with Python dictionaries is trying to access or modify keys that don’t exist in the dictionary. This can raise a KeyError and break your code execution. To handle these situations, the Python defaultdict type is available in the collections module of the standard library.

Understanding the Python defaultdict Type

The Python defaultdict type behaves almost exactly like a regular dictionary, but if you try to access or modify a missing key, it will automatically create the key and generate a default value for it. This makes defaultdict a valuable option for handling missing keys in dictionaries.

Using the Python defaultdict Type

Here are some common use cases for the Python defaultdict type:

Grouping Items

from collections import defaultdict
fruits = [("apple", "red"), ("banana", "yellow"), ("apple", "green"), ("banana", "ripe")]
fruit_groups = defaultdict(list)
for fruit, color in fruits:


{'apple': ['red', 'green'], 'banana': ['yellow', 'ripe']}

Grouping Unique Items

from collections import defaultdict
fruits = [("apple", "red"), ("banana", "yellow"), ("apple", "green"), ("banana", "ripe")]
fruit_groups = defaultdict(set)
for fruit, color in fruits:


{'apple': {'green', 'red'}, 'banana': {'yellow', 'ripe'}}

Counting Items

from collections import defaultdict
fruits = ["apple", "banana", "apple", "banana", "apple"]
fruit_counts = defaultdict(int)
for fruit in fruits:
fruit_counts[fruit] += 1


{'apple': 3, 'banana': 2}

Accumulating Values

from collections import defaultdict
fruits = [("apple", 3), ("banana", 2), ("apple", 5), ("banana", 1)]
total_fruit_count = defaultdict(int)
for fruit, count in fruits:
total_fruit_count[fruit] += count


{'apple': 8, 'banana': 3}

Diving Deeper Into defaultdict

defaultdict vs dict

The main difference between defaultdict and a regular dictionary is that defaultdict automatically generates a default value when accessing a missing key, whereas a regular dictionary raises a KeyError. This can simplify your code and make it more readable.


By default, a defaultdict uses None as its default_factory. However, you can specify any other callable (e.g., int, list, set, lambda functions) as the default_factory argument when creating a defaultdict.

defaultdict vs dict.setdefault()

While both defaultdict and dict.setdefault() can provide default values for missing keys, there is a key difference. dict.setdefault() modifies the original dictionary, whereas defaultdict creates a new key-value pair without modifying the original dictionary.


The __missing__() method is a special method that you can override in a defaultdict. It is called whenever a missing key is accessed, allowing you to define custom behavior.

Emulating the Python defaultdict Type

In situations where you can’t or don’t want to use defaultdict, you can emulate its behavior by subclassing the built-in dict type and overriding the __missing__() method.

Passing Arguments to .default_factory

You can pass arguments to the default_factory when creating a defaultdict. Two common approaches are using lambda functions or functools.partial().


The Python defaultdict type is a powerful tool for handling missing keys in dictionaries. It automatically generates default values for missing keys, simplifying your code and preventing KeyError exceptions. It can be used for grouping, counting, and accumulating operations, among other use cases. Consider using defaultdict whenever you need to work with dictionaries that may contain missing keys.