How to Create Dictionary with Default Value in Python

In Python, you can create a dictionary with a default value by using collections.defaultdict which automatically assigns a default for missing keys, or by using the dict.get() method to provide a default when accessing keys. Both methods help avoid errors when keys are not present.

Syntax

There are two common ways to create a dictionary with default values:

• Using defaultdict: You import defaultdict from collections and provide a function that returns the default value.
• Using dict.get() method: You use get(key, default_value) to safely access keys with a fallback default.

python

from collections import defaultdict

# Using defaultdict
my_dict = defaultdict(lambda: 'default value')

# Using dict.get()
regular_dict = {'a': 1, 'b': 2}
value = regular_dict.get('c', 'default value')

Example

This example shows how defaultdict automatically returns a default value for missing keys, and how dict.get() returns a default without raising an error.

python

from collections import defaultdict

# Create defaultdict with default string
my_dict = defaultdict(lambda: 'Not Found')
my_dict['apple'] = 10

print(my_dict['apple'])   # Existing key
print(my_dict['banana'])  # Missing key returns default

# Using dict.get()
regular_dict = {'apple': 10}
print(regular_dict.get('apple', 'Not Found'))  # Existing key
print(regular_dict.get('banana', 'Not Found')) # Missing key returns default

Output

10
Not Found
10
Not Found

Common Pitfalls

One common mistake is trying to assign a default value directly in a normal dictionary, which does not work automatically for missing keys and raises a KeyError. Another is using mutable default values incorrectly with defaultdict, which can cause unexpected shared state.

Always use a function (like lambda) to provide default values in defaultdict to avoid shared mutable defaults.

python

from collections import defaultdict

# Wrong: Using mutable default directly (shared list)
wrong_dict = defaultdict(list)
wrong_dict['a'].append(1)
wrong_dict['b'].append(2)
print(wrong_dict)  # Works fine here but beware if default is mutable and reused

# Correct: Use lambda to create new list each time
correct_dict = defaultdict(lambda: [])
correct_dict['a'].append(1)
correct_dict['b'].append(2)
print(correct_dict)

Output

{'a': [1], 'b': [2]}
{'a': [1], 'b': [2]}

Quick Reference

Here is a quick summary of methods to create dictionaries with default values:

Method	Description	Example
defaultdict	Creates dictionary with automatic default for missing keys	defaultdict(lambda: 0)
dict.get()	Returns default value when key is missing	my_dict.get('key', default)
setdefault()	Returns value if key exists, else sets and returns default	my_dict.setdefault('key', default)

Key Takeaways

Use collections.defaultdict with a function to create dictionaries with automatic default values.

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

Avoid assigning mutable default values directly in defaultdict to prevent shared state bugs.

dict.setdefault() can also set and return a default value for missing keys.

Normal dictionaries raise KeyError if you access missing keys without a default.