How to Set Default Value in Dictionary Python

In Python, you can set a default value in a dictionary by using the dict.get(key, default) method which returns the default if the key is missing. Alternatively, use collections.defaultdict to automatically assign default values for missing keys.

Syntax

Here are two common ways to set default values in a Python dictionary:

• Using dict.get(key, default): Returns the value for key if it exists, otherwise returns default.
• Using collections.defaultdict(default_factory): Creates a dictionary that automatically assigns a default value from default_factory when a missing key is accessed.

python

value = my_dict.get(key, default_value)

from collections import defaultdict
my_dict = defaultdict(default_factory)

Example

This example shows how to use dict.get() to safely get a value with a default, and how to use defaultdict to automatically assign default values for missing keys.

python

from collections import defaultdict

# Using dict.get() method
my_dict = {'apple': 2, 'banana': 3}
print(my_dict.get('apple', 0))   # Output: 2
print(my_dict.get('orange', 0))  # Output: 0 (default)

# Using defaultdict
fruit_count = defaultdict(int)  # int() returns 0
fruit_count['apple'] = 2
fruit_count['banana'] = 3
print(fruit_count['apple'])    # Output: 2
print(fruit_count['orange'])   # Output: 0 (default automatically assigned)

Output

2
0
2
0

Common Pitfalls

One common mistake is trying to assign a default value by directly accessing a missing key, which raises a KeyError. Another is modifying the dictionary inside a loop without using safe methods.

Always use dict.get() or defaultdict to avoid errors when keys might be missing.

python

my_dict = {'a': 1}

# Wrong way: raises KeyError
# print(my_dict['b'])

# Right way: use get() with default
print(my_dict.get('b', 0))  # Output: 0

# Using defaultdict to avoid KeyError
from collections import defaultdict
my_defaultdict = defaultdict(lambda: 'missing')
my_defaultdict['a'] = 1
print(my_defaultdict['b'])  # Output: 'missing'

Output

0
missing

Quick Reference

Summary of methods to set default values in Python dictionaries:

Method	Description	Example
dict.get(key, default)	Returns value if key exists, else default	my_dict.get('key', 0)
collections.defaultdict(default_factory)	Auto-assigns default for missing keys	defaultdict(int) assigns 0 by default
dict.setdefault(key, default)	Sets default if key missing and returns value	my_dict.setdefault('key', 0)

Key Takeaways

Use dict.get(key, default) to safely access keys with a fallback value.

Use collections.defaultdict to automatically assign default values for missing keys.

Avoid accessing missing keys directly to prevent KeyError exceptions.

dict.setdefault can also set and return a default value if the key is missing.

Choose the method that best fits your use case for cleaner and safer code.