How does exception handling work in Python?

Q: How does exception handling work in Python?

**Target Audience:** Beginners learning Python, and anyone preparing for a Python interview **Difficulty:** Easy **Python version:** Every example was run on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3) > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. Every output block in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First You ask a friend to pick up a parcel from the post office. Most days it just works. But sometimes the office is closed, sometimes the parcel is not there yet, and sometimes your friend gets a flat tyre on the way. A good friend does not silently give up, and does not pretend the parcel was collected. They come back and tell you **what went wrong**, so that you can decide what to do: try tomorrow, send someone else, or give up. And whatever h

The problem

Question presented to candidate: "Walk me through how exception handling works in Python. What do try, except, else and finally each do, in what order do they run, and how does Python decide which except clause handles an error?"

What a strong answer should cover:

An exception is an object (an instance of a class derived from BaseException) that stops normal execution and travels up the call stack until a matching except clause handles it
The four clauses: try holds the risky code, except handles matching errors, else runs only when try raised nothing, finally runs on every way out (normal end, return, or an error passing through)
Matching is by isinstance, clauses are tested top to bottom, the first match wins, so specific classes go before their base classes; a tuple catches several types
Catch Exception, not a bare except: (which also catches KeyboardInterrupt and SystemExit); keep the try block small; use bare raise to re-raise unchanged
The as e name is deleted when the except block ends, and a return inside finally silently discards an in-flight exception
CPython detail: since 3.11 a try costs almost nothing when no exception is raised ("zero-cost" exceptions, via an exception table instead of setup instructions)

Code / implementation expected: Optional — a short try/except/else/finally function showing which clauses run for success and for failure.

exceptions

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners learning Python, and anyone preparing for a Python interview Difficulty: Easy Python version: Every example was run on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3)

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. Every output block in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

You ask a friend to pick up a parcel from the post office. Most days it just works. But sometimes the office is closed, sometimes the parcel is not there yet, and sometimes your friend gets a flat tyre on the way. A good friend does not silently give up, and does not pretend the parcel was collected. They come back and tell you **what went wro

The solution is hidden so you can try first

11 min read · 13 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

Pythontry / except / else / finally: which clause runs when

Practice

PythonThe hierarchy, clause order, the deleted as-name, bare raise and finally traps