How to Swap Keys and Values in Dictionary in Python

To swap keys and values in a Python dictionary, use a dictionary comprehension like {v: k for k, v in original_dict.items()}. This creates a new dictionary where original values become keys and original keys become values.

Syntax

The syntax to swap keys and values in a dictionary uses a dictionary comprehension:

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

Here, original_dict.items() gives pairs of keys and values. The comprehension creates a new dictionary with v as the new key and k as the new value.

python

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

Example

This example shows how to swap keys and values in a dictionary of fruits and their colors.

python

original_dict = {'apple': 'red', 'banana': 'yellow', 'grape': 'purple'}
swapped_dict = {v: k for k, v in original_dict.items()}
print(swapped_dict)

Output

{'red': 'apple', 'yellow': 'banana', 'purple': 'grape'}

Common Pitfalls

One common mistake is when dictionary values are not unique. Since keys must be unique, swapping will lose data if values repeat.

For example, if two keys have the same value, only one will appear in the swapped dictionary.

python

original_dict = {'a': 1, 'b': 1}
swapped_dict = {v: k for k, v in original_dict.items()}
print(swapped_dict)  # Output: {1: 'b'}

# Correct approach if values repeat is to store keys in a list:
from collections import defaultdict
swapped_dict = defaultdict(list)
for k, v in original_dict.items():
    swapped_dict[v].append(k)
print(dict(swapped_dict))  # Output: {1: ['a', 'b']}

Output

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

Quick Reference

Tips for swapping keys and values in Python dictionaries:

• Use dictionary comprehension: {v: k for k, v in dict.items()}
• Ensure values are unique to avoid data loss
• Use defaultdict(list) to handle duplicate values

Key Takeaways

Use dictionary comprehension to swap keys and values easily.

Original dictionary values must be unique to avoid losing data.

For duplicate values, group keys in a list using defaultdict.

Swapping creates a new dictionary; original stays unchanged.

Always check data types to ensure keys are hashable after swap.