Python Comprehensions & Generators

6Comprehensions & Generators

The rules, again

Same game as chapter 1: read the question, commit to a prediction, run it, and only then open the answer. Wrong prediction means the drill goes on your redo list for three days from now.

This chapter is where Python starts feeling like Python. Comprehensions are the surface: one-line list building that every codebase uses. Generators are the depth: functions that pause mid-thought, sequences that never fit in memory, and the one-shot exhaustion trap that produces the quietest bug in the language.

Learn the concepts first

Rusty on the basics? Work through List Comprehensions and Generators on learnpython.org first, then come back and drill.

One idea before you start

Every generator drill in this chapter is the same principle wearing different costumes.

Definition

Lazy evaluation means computing a value only at the moment something asks for it. A list holds all its values right now; a generator holds a recipe and cooks one value per request. That is why generators handle huge or infinite sequences in constant memory, and why they are empty after one pass: the recipe has been used up.

The drills

Click a question to open it. The answer stays hidden until you ask for it. Predict first.

14 drills. Predict first, then open the answer

Want more drills?

When these stop surprising you, let an AI generate fresh ones. Copy this prompt into ChatGPT, Claude, or any assistant you like:

You are my Python drill coach. Quiz me on: list, dict, and set
comprehensions, nested comprehensions, generator expressions, yield,
next() and StopIteration, generator exhaustion, and any()/all().
 
Rules:
- Ask ONE question at a time. Show a short code snippet and ask me to
  predict the output (or the error) before running it.
- Wait for my answer. Do not reveal anything until I reply.
- If I am right, say so in one line and raise the difficulty slightly.
- If I am wrong, explain the why in 2-3 sentences, then give me a
  similar question later to check I really got it.
- Prefer tricky edge cases over textbook questions: if-filter vs
  if-else transform position, flattening order of nested fors, {} being
  a dict, consuming a generator twice, print statements inside a
  generator function, all([]) being True.
- Keep score. After 10 questions, list the concepts I missed so I can
  add them to my redo list.
 
Start with a medium question.

The predict-first rule matters more than the tool. Whatever generates the questions, always commit to an answer before you look.

Done?

Score yourself honestly and queue up your misses. The two career-savers here: drill 7 (nothing runs until you ask) and drill 8 (one pass and it is gone). Between them they explain most "my data disappeared" bugs involving generators, and drill 13's all([]) is a real production incident wearing a quiz costume.

Next chapter: object-oriented Python, where self stops being mysterious and dunder methods let your objects speak the language's native tongue.