hypothesis

Hypothesis - Property-Based Testing

Property-based testing framework that generates test cases automatically, finds minimal failing examples through shrinking, and verifies invariants.

Official Docs: https://hypothesis.readthedocs.io/en/latest/

Key Features:

• Automatic test data generation from strategies
• Minimal failing example shrinking
• Stateful testing with rule-based state machines
• pytest integration
• Deterministic reproducibility

Quick Start

from hypothesis import given
from hypothesis import strategies as st


@given(st.integers())
def test_property(x):
    """Test properties that should always hold"""
    assert abs(x) >= 0


@given(st.lists(st.integers()))
def test_list_property(lst):
    sorted_lst = sorted(lst)
    assert len(sorted_lst) == len(lst)
    # Check monotonic property
    for i in range(len(sorted_lst) - 1):
        assert sorted_lst[i] <= sorted_lst[i + 1]

Strategies

Full reference: https://hypothesis.readthedocs.io/en/latest/data.html

Common strategies:

• Primitives: st.integers(), st.floats(), st.text(), st.booleans()
• Collections: st.lists(), st.dictionaries(), st.tuples(), st.sets()
• Dates/Times: st.dates(), st.datetimes(), st.timedeltas()
• Combinators: st.one_of(), st.sampled_from(), st.recursive()
• Type-based: st.from_type(MyClass)

Composite Strategies

from hypothesis import strategies as st
from hypothesis.strategies import composite


@composite
def user_strategy(draw):
    age = draw(st.integers(min_value=18, max_value=100))
    name = draw(st.text(min_size=1))
    return {"name": name, "age": age, "is_adult": age >= 18}


@given(user_strategy())
def test_user(user):
    assert user["is_adult"] == (user["age"] >= 18)

Strategy Combinators

st.integers().filter(lambda x: x % 2 == 0)  # Filter
st.integers().map(str)  # Transform
st.one_of(st.integers(), st.text())  # Choose between strategies
st.sampled_from([1, 2, 3, 4, 5])  # Pick from collection
st.from_type(MyClass)  # Infer from type hints
st.builds(MyClass, arg1=st.integers())  # Build instances