Check in lists

fruits = ['apple', 'banana', 'cherry']

if 'apple' in fruits:
    print("Found apple")

if 'orange' not in fruits:
    print("No orange")

Check in strings

text = "Hello World"

if 'World' in text:
    print("Found 'World'")

if 'x' not in text:
    print("No 'x' in text")

Check in dictionaries

person = {'name': 'Alice', 'age': 30}

# Check keys:
if 'name' in person:
    print("Has name key")

# Check values (less common):
if 'Alice' in person.values():
    print("Alice is a value")

Check in sets

numbers = {1, 2, 3, 4, 5}

if 3 in numbers:
    print("3 is in the set")

# Very fast for sets (O(1) average)

With loops

# Common pattern:
items = [1, 2, 3, 4, 5]
target = 3

if target in items:
    print(f"Found {target} at index {items.index(target)}")
else:
    print("Not found")

Performance note

  • Lists: O(n) — checks each item
  • Sets/Dicts: O(1) average — very fast
  • Strings: O(n) — checks each character