What is EAFP vs LBYL in Python? | Interview Question

Q: What is EAFP vs LBYL in Python?

**Target Audience:** Python developers who know try/except and want to know when to check first and when to just try **Difficulty:** Medium **Python version:** Checked 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 want to borrow a colleague's stapler. One way: walk to their desk, check whether the stapler is there, walk back, ask them by email whether you may borrow it, wait, walk back to the desk... and find that someone else took it while you were waiting. The other way: walk over, pick it up, and if it is not there, deal with that ("never mind, I will use the one at reception"). The first style checks every condition in advance an

The problem

Question presented to candidate: "People say Python prefers 'easier to ask forgiveness than permission'. What do EAFP and LBYL mean, why is EAFP considered Pythonic, and are there cases where you would still check first?"

What a strong answer should cover:

LBYL ("look before you leap") tests preconditions with if before acting; EAFP ("easier to ask for forgiveness than permission") just performs the operation and handles the specific exception with try/except
Correctness: a check and the operation can disagree ("²".isdigit() is true but int("²") fails), and between a check and the use the world can change (time-of-check to time-of-use races on files, or dict keys across threads, as the Python glossary notes)
EAFP fits duck typing: try the operation instead of testing the type; hasattr itself works by calling getattr and catching AttributeError
Cost: entering try is nearly free in CPython 3.11+, but raising and catching is several times slower than an if, so LBYL (or dict.get) wins when failure is the common case
When LBYL is right: validating all user input up front, avoiding partial side effects, and conditions that are cheap and cannot change; keep EAFP try blocks to the single failing line

Code / implementation expected: Optional — the same lookup or file read written both ways, with the race or the edge case that breaks the LBYL version.

eafp
lbyl

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: Python developers who know try/except and want to know when to check first and when to just try Difficulty: Medium Python version: Checked 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 want to borrow a colleague's stapler. One way: walk to their desk, check whether the stapler is there, walk back, ask them by email whether you may borrow it, wait, walk back to the desk... and find that someone else took it while you were waiting. The other way: walk over, pick it up, and if it is not there, deal with that ("

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10 min read · 10 sections

Run the code

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

PythonParsing, dict lookups, duck typing and hasattr: EAFP vs LBYL

Practice

PythonCheck-then-use races with files, atomic create, and a too-wide try

Practice

Discussion

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