Lists and Dictionaries

Lists

Ordered, mutable sequence. Create with [] or list().

python
nums = [1, 2, 3]
nums.append(4)
nums[0]   # 1
nums[-1]  # 4 (last)
len(nums) # 4

Slicing: nums[1:3] is from index 1 up to (not including) 3. List comprehensions:

python
squares = [x ** 2 for x in range(5)]  # [0, 1, 4, 9, 16]

Dictionaries

Key-value pairs. Create with {} or dict().

python
user = {\"name\": \"Alice\", \"age\": 25}
user[\"name\"]   # Alice
user.get(\"city\", \"Unknown\")  # default if key missing
user[\"city\"] = \"NYC\"

Iterate: for k, v in user.items(): or for k in user:.

Tuples and sets

• Tuples – immutable: t = (1, 2).
• Sets – unique, unordered: s = {1, 2, 2} gives {1, 2}.