Python Error Handling & Context Managers

8Error Handling & Context Managers

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.

Error handling is where code quality becomes visible. Anyone can write the happy path; the difference between junior and senior shows in what happens when int() gets None, when the file is missing, when the user hits Ctrl+C. Python gives you sharp tools here, a try with four clauses, exception chaining, the with statement, and each has a trap the drills below will spring on you.

Learn the concepts first

Rusty on the basics? Work through Exception Handling on learnpython.org first, then come back and drill.

One idea before you start

Python's error culture differs from most languages, and one acronym captures it.

Definition

Easier to Ask Forgiveness than Permission: attempt the operation and handle the exception, instead of checking whether it would succeed first. The opposite style, Look Before You Leap, has a gap between the check and the act where the world can change. Python culture leans EAFP, which is why its exception machinery is so developed.

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: try/except/else/finally,
exception hierarchies and handler order, catching multiple types,
re-raising (bare raise, raise from), custom exceptions, EAFP vs LBYL,
the with statement, and writing context managers.
 
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: except Exception
  before except ValueError, return inside finally, using e after the
  except block, KeyboardInterrupt vs except Exception, __exit__
  returning True, assert under python -O.
- 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. Drill 5 (return inside finally) and drill 14 (assert stripped by -O) are the two that pass code review at most companies and then cost real money. If you predicted both, your error handling instincts are production-grade.

Next chapter: modules and files, where imports run code you did not ask for and pathlib finally makes paths pleasant.