Dictionary & Set Comprehensions

Syntax

{key_expr: value_expr for item in iterable}

Examples

words = ["apple", "banana", "cherry"]

lengths = {word: len(word) for word in words}
print(lengths)  # {'apple': 5, 'banana': 6, 'cherry': 6}

scores = {"Alice": 85, "Bob": 55, "Charlie": 92, "Dave": 40}

passing = {name: score for name, score in scores.items() if score >= 60}
print(passing)  # {'Alice': 85, 'Charlie': 92}

words = ["Hello", "world", "HELLO", "World"]

unique_lower = {word.lower() for word in words}
print(unique_lower)  # {'hello', 'world'}

country_codes = {"UK": "United Kingdom", "US": "United States"}

code_lookup = {name: code for code, name in country_codes.items()}
print(code_lookup)  # {'United Kingdom': 'UK', 'United States': 'US'}

Best practices

• Use dictionary comprehensions to build lookups from another dictionary or a list of tuples, instead of a manual loop with repeated dict[key] = value
• Remember set comprehensions automatically deduplicate results - useful when transforming values that may collide
• Keep the key and value expressions simple - extract complex logic into a helper function if it gets hard to read
• Watch out for accidentally overwriting keys when the key expression isn't unique across items

At a glance

Purpose

Scripting and general-purpose applications

File extension

.py

Runs in

Python interpreter

Usually used with

Python standard library and packages

Specifications & further reading

• Python language reference

Related Python documentation