How to Create Dictionary from List in Python: Simple Guide

You can create a dictionary from a list in Python by using dict() with a list of key-value pairs or by using a dictionary comprehension to assign keys and values. For example, use dict([(key, value), ...]) or {key: value for item in list} to build the dictionary.

Syntax

There are two common ways to create a dictionary from a list:

• Using dict() with a list of tuples: Each tuple contains a key and a value.
• Using dictionary comprehension: You write a short expression to generate key-value pairs from the list.

python

dict([(key1, value1), (key2, value2), ...])

# or

{key_expression: value_expression for item in list}

Example

This example shows how to create a dictionary from a list of numbers where keys are the numbers and values are their squares.

python

numbers = [1, 2, 3, 4]
squares_dict = {num: num ** 2 for num in numbers}
print(squares_dict)

Output

{1: 1, 2: 4, 3: 9, 4: 16}

Common Pitfalls

Common mistakes include:

• Trying to convert a list of single values directly to a dictionary without pairing keys and values.
• Using lists with duplicate keys, which will overwrite previous values.
• Not using tuples or pairs inside dict(), which causes errors.

python

wrong_list = [1, 2, 3]
# This will cause an error because items are not key-value pairs
# my_dict = dict(wrong_list)  # Raises TypeError

# Correct way with pairs:
pairs = [(1, 'a'), (2, 'b'), (3, 'c')]
my_dict = dict(pairs)
print(my_dict)

Output

{1: 'a', 2: 'b', 3: 'c'}

Quick Reference

Summary tips for creating dictionaries from lists:

• Use dict() with a list of (key, value) tuples.
• Use dictionary comprehension for flexible key-value creation.
• Ensure keys are unique to avoid overwriting.
• Remember dictionary keys must be immutable types like strings, numbers, or tuples.

Key Takeaways

Use dict() with a list of (key, value) pairs to create a dictionary from a list.

Dictionary comprehension offers a concise way to build dictionaries from lists.

Keys must be unique and immutable to avoid errors or overwriting values.

Avoid passing a list of single values directly to dict(), it requires pairs.

Dictionary comprehension allows custom logic for keys and values from list items.