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Write type hints for strategies
===============================
Hypothesis provides type hints for all strategies and functions which return a strategy:
.. code-block:: python
from hypothesis import strategies as st
reveal_type(st.integers())
# SearchStrategy[int]
reveal_type(st.lists(st.integers()))
# SearchStrategy[list[int]]
|SearchStrategy| is the type of a strategy. It is parametrized by the type of the example it generates. You can use it to write type hints for your functions which return a strategy:
.. code-block:: python
from hypothesis import strategies as st
from hypothesis.strategies import SearchStrategy
# returns a strategy for "normal" numbers
def numbers() -> SearchStrategy[int | float]:
return st.integers() | st.floats(allow_nan=False, allow_infinity=False)
It's worth pointing out the distinction between a strategy, and a function that returns a strategy. |st.integers| is a function which returns a strategy, and that strategy has type ``SearchStrategy[int]``. The function ``st.integers`` therefore has type ``Callable[..., SearchStrategy[int]]``, while the value ``s = st.integers()`` has type ``SearchStrategy[int]``.
Type hints for |st.composite|
-----------------------------
When writing type hints for strategies defined with |st.composite|, use the type of the returned value (not ``SearchStrategy``):
.. code-block:: python
@st.composite
def ordered_pairs(draw) -> tuple[int, int]:
n1 = draw(st.integers())
n2 = draw(st.integers(min_value=n1))
return (n1, n2)
Type variance of |SearchStrategy|
---------------------------------
|SearchStrategy| is covariant, meaning that if ``B < A`` then ``SearchStrategy[B] < SearchStrategy[A]``. In other words, the strategy ``st.from_type(Dog)`` is a subtype of the strategy ``st.from_type(Animal)``.
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