Simon Willison’s Weblog

Hypothesis is now thread-safe (via) Hypothesis is a property-based testing library for Python. It lets you write tests like this one:

from hypothesis import given, strategies as st

@given(st.lists(st.integers()))
def test_matches_builtin(ls):
    assert sorted(ls) == my_sort(ls)

This will automatically create a collection of test fixtures that exercise a large array of expected list and integer shapes. Here's a Gist demonstrating the tests the above code will run, which include things like:

[]
[0]
[-62, 13194]
[44, -19562, 44, -12803, -24012]
[-7531692443171623764, -109369043848442345045856489093298649615]

Hypothesis contributor Liam DeVoe was recently sponsored by Quansight to add thread safety to Hypothesis, which has become important recently due to Python free threading:

While we of course would always have loved for Hypothesis to be thread-safe, thread-safety has historically not been a priority, because running Hypothesis tests under multiple threads is not something we see often.

That changed recently. Python---as both a language, and a community---is gearing up to remove the global interpreter lock (GIL), in a build called free threading. Python packages, especially those that interact with the C API, will need to test that their code still works under the free threaded build. A great way to do this is to run each test in the suite in two or more threads simultaneously. [...]

Nathan mentioned that because Hypothesis is not thread-safe, Hypothesis tests in community packages have to be skipped when testing free threaded compatibility, which removes a substantial battery of coverage.

Now that Hypothesis is thread-safe another blocker to increased Python ecosystem support for free threading has been removed!