How to Filter Dictionary by Key in Python: Simple Guide

To filter a dictionary by key in Python, use dictionary comprehension with a condition on keys, like {k: v for k, v in my_dict.items() if k in keys_to_keep}. This creates a new dictionary containing only the keys you want.

Syntax

The basic syntax to filter a dictionary by key uses dictionary comprehension:

{key: value for key, value in original_dict.items() if condition_on_key}

Here:

• original_dict.items() gives pairs of keys and values.
• key: value builds the new dictionary entries.
• if condition_on_key filters keys based on your condition.

python

{k: v for k, v in original_dict.items() if k in keys_to_keep}

Example

This example shows how to keep only certain keys from a dictionary.

python

my_dict = {'apple': 1, 'banana': 2, 'cherry': 3, 'date': 4}
keys_to_keep = {'banana', 'date'}
filtered_dict = {k: v for k, v in my_dict.items() if k in keys_to_keep}
print(filtered_dict)

Output

{'banana': 2, 'date': 4}

Common Pitfalls

Common mistakes when filtering dictionaries by key include:

• Using dict.keys() without .items(), which gives keys only, not key-value pairs.
• Modifying the dictionary while iterating over it, which causes errors.
• Forgetting that dictionary comprehension creates a new dictionary, so the original stays unchanged.

python

my_dict = {'a': 1, 'b': 2, 'c': 3}
# Wrong: trying to filter keys without items()
# filtered = {k: my_dict[k] for k in my_dict.keys() if k == 'a'}  # Works but less efficient

# Right: use items() for direct access
filtered = {k: v for k, v in my_dict.items() if k == 'a'}
print(filtered)

Output

{'a': 1}

Quick Reference

Tips for filtering dictionaries by key:

• Use dictionary comprehension with .items() for clean and efficient filtering.
• Check if keys match your condition inside the if clause.
• Remember the original dictionary is not changed; you get a new filtered dictionary.

Key Takeaways

Use dictionary comprehension with .items() to filter by key efficiently.

The condition inside the comprehension controls which keys to keep.

Filtering creates a new dictionary; the original stays unchanged.

Avoid modifying a dictionary while iterating over it.

Check your key condition carefully to avoid unexpected results.