proptest

proptest is a property testing framework for Mojo (i.e., the QuickCheck family) inspired by the Hypothesis framework for Python, with ideas from Rust’s proptest and quickcheck, Haskell’s QuickCheck, and C++’s RapidCheck.

With proptest, you write tests that should pass for every input in a range you describe, and proptest chooses which of those inputs to try, including edge cases you might not have thought of. When it finds an input that fails, it shrinks it to a minimal failing example before reporting it:

from std.testing import assert_true
from proptest import given, lists, integers

def my_sort(xs: List[Int]) -> List[Int]:
    var ys = xs.copy()
    sort(ys)
    return ys^

def test_sort_is_ordered() raises:
    def prop(xs: List[Int]) raises:
        var ys = my_sort(xs)
        for i in range(1, len(ys)):
            assert_true(ys[i - 1] <= ys[i])
    given(lists(integers()), prop)

def main() raises:
    test_sort_is_ordered()

proptest is built on Hypothesis’s Conjecture engine. Strategies don’t generate values directly: they parse them from a recorded sequence of choices, and the shrinker simplifies that sequence. Shrinking therefore works for every strategy, including the ones you write yourself, without any shrinking code of your own. The other libraries contributed most of the typed API: inferring inputs from a property’s parameter types, constraint types such as Positive[T], coverage checking, and model-based testing.

How this book is organized

The book follows the layout of the Hypothesis documentation:

• Quickstart: the essentials on one page.
• Tutorial: an introduction to proptest, from a first test through strategies, settings, the example database, and stateful testing. Start here if you’re new to property-based testing.
• How-to guides: recipes for particular tasks, such as checking the distribution of your inputs or debugging a failure.
• Explanations: how proptest works, and why it is the way it is.
• Reference: every test function, setting, and strategy.

Where proptest behaves differently from Hypothesis, the book says so. The differences document in the repository has the full list, with the reason for each.

Requirements

proptest needs Mojo 1.3 or later. Until Mojo 1.3.0 is released, that means the nightly channel. See Introduction to proptest for installation.