Minimal Reproducible Example
The smallest code that still shows the bug.
Also known as: MRE, minimal reproducible example, MCVE, minimal repro, reproducible example, SSCCE
A minimal reproducible example is the smallest piece of code that still shows your problem, and that someone else can run as-is.
Why bother
- Making one often solves the bug. When you strip everything unrelated away, the cause usually shows itself, or the bug disappears and you learn what was involved.
- It’s what helpers need. Nobody can debug your 5,000-line app from a screenshot. A 20-line example gets answers on forums and issue trackers, and it’s what library maintainers ask for.
How to make one
- Start from the failing code and remove things until the problem goes away, then put back the last thing you removed. Delete features, pages, packages, config.
- Replace real stuff with fakes: hard-coded data instead of a database; a stub instead of a network call.
- Make it self-contained: includes the imports, the input data and the exact command to run.
- State expected vs actual: “I expected
[1, 2], I get[2, 1]”, plus language/library versions.
# Example of a minimal repro for a surprising result
items = [3, 1, 2]
result = sorted(items, key=lambda x: -x)
print(result) # expected [1, 2, 3], got [3, 2, 1]: the key negates the value(That one’s explained by the code itself, which is exactly the benefit of shrinking it.)
A common mistake
Posting only the error line, or the whole project. Aim in between: complete enough to run, small enough to read. Remove secrets and private data first. Also see binary search debugging for halving the problem.