Week 3: For Loops, Lists, and Tuples

Course: Jan 2026 - Python Focus: Iteration over sequences, collection behavior, and nested repetition.

1. Core idea

What to remember

• for iterates over items, not conditions.
• range(n) produces 0 to n - 1.
• Lists are mutable.
• Tuples are immutable.
• Nested loops multiply work, so count them carefully.

2. Common traps

• break stops the loop immediately.
• continue skips the rest of the current turn.
• A list slice returns a list.
• A tuple cannot be edited in place.
• Off-by-one errors often hide in range(start, stop).

3. Speedrun pattern

1. Is this counting iterations or reading a collection?
2. Does the code mutate the list?
3. Is the question about the whole grid, or one cell at a time?

4. Micro revision

• Use for for fixed iteration.
• Use while for unknown iteration count.
• Use lists for changeable collections.
• Use tuples for fixed collections and stable keys.

Assignment anchor

• Week 3 Graded Assignment 3

Navigation

• Previous: Week 2 Notes
• Next: Week 4 Notes