Property-based tests for the Python standard library (and builtins)
Find and fix bugs in Python, before they ship to users.
CPython's existing test suite is good, but bugs still slip through occasionally. We think that using property-based testing tools - i.e. Hypothesis - can help with this. They're no magic bullet, but computer-assisted testing techniques routinely try inputs that humans wouldn't think of (or bother trying), and turn up bugs that humans missed.
- New code can be developed with property-based tests - some already is, but adding it to CPython CI would catch bugs.
- Tests for existing code can be augumented with property-based tests where
this seems particularly valuable - e.g.
tests/test_source_code.pyin this repo has discovered several bugs.
- Property-based testing can be adopted incrementally. Replacing existing tests is an explicit non-goal of this project.
PyPy already uses Hypothesis, and sharing as much of the test suite as possible between implementations would be great. How this would work depends largely on CPython's decisions, though.
By contributing to this repository, you agree to license the contributed code under user's choice of the Mozilla Public License Version 2.0, and the Apache License 2.0.
This dual-licence is intended to make it as easy as possible for the tests in this repository to be used upstream by the CPython project, other implementations of Python, and the Hypothesis project and ecosystem.
To run the tests against the current version of Python:
pip install -r requirements.txt(or
python -m unittest
For development, we use
to manage an extensive suite of auto-formatters and linters, so:
pip install tox
will set up a virtualenv for you, install everything, and finally run the formatters, linters, and test suite.
Bugs found via this specific project:
- BPO-40661, a segfault in the new parser, was given maximum priority and blocked the planned release of CPython 3.9 beta1.
tokenize.untokenizedoes not round-trip as documented. Nor, for that matter, do the
- PEP-615 (zoneinfo)
folddetection failed for early transitions when the number of elapsed seconds is too large to fit in a C integer; and a
foldinconsistency where first offset handling was broken in the C extension.
- BPO-40668, catastrophic loss of precision when attempting to round-trip YIQ-RGB-YIQ
colorsysmodule - more than 10% of the possible range. (via #13)
- Hypothesis' website, documentation, GitHub repo
- Introductory articles, simple properties, metamorphic properties
- Related thoughts from hardware testing and verification, testing a screencast editor, PBT in Erlang / Elixr, testing C code using Hypothesis
python-aflor OSS-FUZZ could work very nicely with Hypothesis' fuzz support
hypothesmithgenerates syntatically-valid but weird Python source code (e.g. BPO-38953 or psf/black#970). Using a syntax tree for semantic validity is the logical next step.