dict vs defaultdict

Quality Score

Overall Score: 8.1/10 ✅ Good

• Technical Accuracy: 28/35
• Code Quality: 20/25
• Educational Value: 20/25
• Documentation: 13/15

Last reviewed: June 22, 2026

Inspect the difference between a dict and a defaultdict in Python.

Access dict by key with square brackets

The easiest way to access a value inside dictionary is with [key] syntax. But be careful, if the key does not exist, you will get a KeyError.

src.beginner.dict_vs_defaultdict.get_value_from_dict_with_square_brackets(my_dict, key)

Get value from a dict using square brackets.

This demonstrates direct dictionary access using square bracket notation. This approach raises a KeyError if the key doesn't exist, which can be useful when you want to ensure the key is present.

Parameters:

Name	Type	Description	Default
my_dict	dict[str, str]	Dictionary with string keys and string values to search for the value.	required
key	str	Key to look up in the dictionary.	required

Returns:

Type	Description
str	The value associated with the key in the dictionary.

Raises:

Type	Description
KeyError	If the key is not found in the dictionary.

Source code in src/beginner/dict_vs_defaultdict/dict_vs_defaultdict.py

def get_value_from_dict_with_square_brackets(
    my_dict: dict[str, str],
    key: str,
) -> str:
    """Get value from a dict using square brackets.

    This demonstrates direct dictionary access using square bracket notation.
    This approach raises a KeyError if the key doesn't exist, which can be
    useful when you want to ensure the key is present.

    Args:
        my_dict: Dictionary with string keys and string values
            to search for the value.
        key: Key to look up in the dictionary.

    Returns:
        The value associated with the key in the dictionary.

    Raises:
        KeyError: If the key is not found in the dictionary.
    """
    return my_dict[key]

Access dict by key with get method

Using get method is another option. If key is not present in dict, None (or custom value) is returned.

src.beginner.dict_vs_defaultdict.get_value_from_dict_with_get(my_dict, key, default=None)

Get value from a dict using get method.

The get() method provides a safe way to access dictionary values, returning a default value instead of raising KeyError when the key is not found. This is preferred when missing keys are expected.

Parameters:

Name	Type	Description	Default
my_dict	dict[str, str]	Dictionary with string keys and string values to search for the value.	required
key	str	Key to look up in the dictionary.	required
default	str | None	Value to return if key is not found. Defaults to None.	None

Returns:

Type	Description
str | None	The value associated with the key, or the default value if the
str | None	key is not found.

Source code in src/beginner/dict_vs_defaultdict/dict_vs_defaultdict.py

def get_value_from_dict_with_get(
    my_dict: dict[str, str],
    key: str,
    default: str | None = None,
) -> str | None:
    """Get value from a dict using get method.

    The get() method provides a safe way to access dictionary values,
    returning a default value instead of raising KeyError when the key
    is not found. This is preferred when missing keys are expected.

    Args:
        my_dict: Dictionary with string keys and string values
            to search for the value.
        key: Key to look up in the dictionary.
        default: Value to return if key is not found. Defaults to None.

    Returns:
        The value associated with the key, or the default value if the
        key is not found.
    """
    return my_dict.get(key, default)

Use defaultdict

defaultdict enables a dict with a default value, even if requested with square brackets. When setting defaultdict, you can send as first argument (default_factory) a function that will be called when key is not present in dict . String, int, list, None... any type you want. If you don't set default_factory, KeyError will be raised if key is not present.

src.beginner.dict_vs_defaultdict.get_value_from_defaultdict(my_dict, key, default=None)

Get value from a defaultdict using square brackets.

Demonstrates how defaultdict automatically provides default values for missing keys. When a lambda function is passed to defaultdict, it's called to generate the default value whenever a missing key is accessed.

Parameters:

Name	Type	Description	Default
my_dict	dict[str, str]	Regular dictionary with string keys and string values to convert to defaultdict.	required
key	str	Key to look up in the defaultdict.	required
default	str | None	Value that the lambda function should return for missing keys. Defaults to None.	None

Returns:

Type	Description
str | None	The value associated with the key, or the default value if the
str | None	key was not in the original dictionary.

Source code in src/beginner/dict_vs_defaultdict/dict_vs_defaultdict.py

def get_value_from_defaultdict(
    my_dict: dict[str, str],
    key: str,
    default: str | None = None,
) -> str | None:
    """Get value from a defaultdict using square brackets.

    Demonstrates how defaultdict automatically provides default values for
    missing keys. When a lambda function is passed to defaultdict, it's
    called to generate the default value whenever a missing key is accessed.

    Args:
        my_dict: Regular dictionary with string keys and string values
            to convert to defaultdict.
        key: Key to look up in the defaultdict.
        default: Value that the lambda function should return for missing
            keys. Defaults to None.

    Returns:
        The value associated with the key, or the default value if the
        key was not in the original dictionary.
    """
    default_dict: defaultdict[str, str | None] = defaultdict(lambda: default)
    default_dict.update(**my_dict)
    return default_dict[key]

Performance comparison

There is a great performance in using defaultdict vs get

from timeit import timeit

print("Get missing default dict:", timeit(
    stmt="default_dict.get('key')",
    setup="default_dict = {}",
    number=5000000)
)
Get missing default dict: 0.1267744980000316

print("Get missing collection default dict:", timeit(
    stmt="default_dict['key']",
    setup="from collections import defaultdict; default_dict = defaultdict(lambda: None)",  # noqa
    number=5000000)
)
Get missing collection default dict: 0.0706390929999543

Common pitfalls

defaultdict creates a new entry in the dictionary when you access a missing key. This can lead to unexpected behavior if you are not careful.