hypothesis plugin¶

We provide several extra features for Hypothesis users. And encourage to use it together with returns.

Installation¶

You will need to install hypothesis separately. It is not bundled with returns.

We also require anyio package for this plugin to work with async laws.

hypothesis entrypoint¶

We support a hypothesis entrypoint that is executed on hypothesis import.

There we are registering all our containers as strategies. So, you don’t have to. Example:

from returns.result import Result
from hypothesis import strategies as st

assert st.from_type(Result).example()

This means you can use Result, Maybe, etc. in your own property tests, and hypothesis will generate values for them as expected.

check_all_laws¶

We also provide a very powerful mechanism of checking defined container laws. It works in a combination with “Laws as Values” feature we provide in the core.

from returns.contrib.hypothesis.laws import check_all_laws
from your.module import YourCustomContainer

check_all_laws(YourCustomContainer)

This one line of code will generate ~100 tests for all defined laws in both YourCustomContainer and all its super types, including our internal ones and user-defined ones.

We also provide a way to configure the checking process with settings_kwargs:

check_all_laws(YourCustomContainer, settings_kwargs={'max_examples': 500})

This will increase the number of generated test to 500. We support all kwargs from @settings, see @settings docs.

You can also change how hypothesis creates instances of your container. By default, we use .from_value, .from_optional, and .from_failure if we are able to find them.

But, you can also pass types without these methods, but with __init__ defined:

from typing import Callable, TypeVar, final
from returns.interfaces.mappable import Mappable1
from returns.primitives.container import BaseContainer
from returns.primitives.hkt import SupportsKind1

_ValueType = TypeVar('_ValueType')
_NewValueType = TypeVar('_NewValueType')

@final
class Number(
    BaseContainer,
    SupportsKind1['Number', _ValueType],
    Mappable1[_ValueType],
):
    def __init__(self, inner_value: _ValueType) -> None:
        super().__init__(inner_value)

    def map(
        self,
        function: Callable[[_ValueType], _NewValueType],
    ) -> 'Number[_NewValueType]':
        return Number(function(self._inner_value))

# We want to allow ``__init__`` method to be used:
check_all_laws(Number, use_init=True)

As you see, we don’t support any from methods here. But, __init__ would be used to generate values thanks to use_init=True.

By default, we don’t allow to use __init__, because there are different complex types like Future, ReaderFutureResult, etc that have complex __init__ signatures. And we don’t want to mess with them.

Warning

Checking laws is not compatible with pytest-xdist, because we use a lot of global mutable state there. Please, use returns_lawful marker to exclude them from pytest-xdist execution plan.

Registering Custom Strategies when Checking Laws¶

hypothesis works by looking up strategies for the provided type annotations. Given that the types provided by returns are very complicated and not really native to Python, they may not be understood by hypothesis, and you may get runtime exceptions such as ResolutionFailed.

In such cases, you may want to register custom strategies for types for which hypothesis does not find any strategies.

The main use case is registering a custom strategy to generate your container when running its laws:

from hypothesis import strategies as st

check_all_laws(Number, container_strategy=st.builds(Number, st.integers()))

You can also register strategies for other types:

from hypothesis import strategies as st

check_all_laws(
    Number,
    container_strategy=st.builds(Number, st.integers()),
    type_strategies={Foo: st.builds(Foo, st.text())},
)

These custom strategies will be used only when running the tests generated by the check_all_laws call above. They will have no effect on any other property tests that involve the same types. You cannot use this argument together with use_init.

Registering Custom Strategies outside Law Tests¶

We provide a utility function to create hypothesis strategy from any container: strategy_from_container.

You can use it to register your own containers.

from hypothesis import strategies as st
from returns.contrib.hypothesis.containers import strategy_from_container

st.register_type_strategy(
    YourContainerClass,
    strategy_from_container(YourContainerClass),
)

You can also pass use_init keyword argument if you wish to use __init__ method to instantiate your containers. Turned off by default. Example:

st.register_type_strategy(
    YourContainerClass,
    strategy_from_container(YourContainerClass, use_init=True),
)

Or you can write your own hypothesis strategy. It is also fine.

Warning::: Avoid directly registering your container’s strategy with hypothesis using st.register_type_strategy. Because of the way we emulate higher-kinded types, hypothesis may mistakenly use the strategy for other incompatible containers and cause spurious test failures. We specify how to do it just in case you need it and you know what you’re doing.

API Reference¶

Types we have already registered for you¶

Used to register all our types as hypothesis strategies.

See: https://hypothesis.readthedocs.io/en/latest/strategies.html

But, beware that we only register concrete types here, interfaces won’t be registered!

DSL to register custom containers¶

strategy_from_container(container_type, *, use_init=False)[source]¶

Creates a strategy from a container type.

Basically, containers should not support __init__ even when they have one. Because, that can be very complex: for example FutureResult requires Awaitable[Result[a, b]] as an __init__ value.

But, custom containers pass use_init if they are not an instance of ApplicativeN and do not have a working .from_value method. For example, pure MappableN can do that.

We also try to resolve generic arguments. So, Result[_ValueType, Exception] will produce any value for success cases and only exceptions for failure cases.

Parameters:

container_type (type[Lawful])
use_init (bool)

Return type:

Callable[[type], SearchStrategy]

DSL to define laws¶

        classDiagram
  Generic <|-- Law1
  Generic <|-- Law2
  Generic <|-- Law3
  Generic <|-- Lawful
  Immutable <|-- Law
  Law <|-- Law1
  Law <|-- Law2
  Law <|-- Law3

law_definition¶: Special alias to define laws as functions even inside a class

class Law(function)[source]¶

Bases: Immutable

Base class for all laws. Does not have an attached signature.

Should not be used directly. Use Law1, Law2 or Law3 instead.

definition: Callable¶: Function used to define this law.

property name: str¶: Returns a name of the given law. Basically a name of the function.

final class Law1(function)[source]¶

Bases: Law, Generic[_TypeArgType1, _ReturnType]

Law definition for functions with a single argument.

Parameters:: function (Callable[[TypeVar(_TypeArgType1)], TypeVar(_ReturnType)])

final class Law2(function)[source]¶

Bases: Law, Generic[_TypeArgType1, _TypeArgType2, _ReturnType]

Law definition for functions with two arguments.

Parameters:: function (Callable[[TypeVar(_TypeArgType1), TypeVar(_TypeArgType2)], TypeVar(_ReturnType)])

final class Law3(function)[source]¶

Bases: Law, Generic[_TypeArgType1, _TypeArgType2, _TypeArgType3, _ReturnType]

Law definition for functions with three argument.

Parameters:: function (Callable[[TypeVar(_TypeArgType1), TypeVar(_TypeArgType2), TypeVar(_TypeArgType3)], TypeVar(_ReturnType)])

class Lawful[source]¶

Bases: Generic[_Caps]

Base class for all lawful classes.

Allows to smartly collect all defined laws from all parent classes.

final classmethod laws()[source]¶

Collects all laws from all parent classes.

Algorithm: :rtype: dict[type[Lawful], Sequence[Law]]

First, we collect all unique parents in __mro__
Then we get the laws definition from each of them
Then we structure them in a type: its_laws way

class LawSpecDef[source]¶

Bases: object

Base class for all collection of laws aka LawSpecs.

Plugin internals¶

final class Settings(settings_kwargs, use_init, container_strategy, type_strategies)[source]¶

Bases: object

Settings for the law tests.

This sets the context for each generated law test, by temporarily registering strategies for various types and passing any hypothesis settings.

Any settings passed by the user will override the value from default_settings().

Parameters:

settings_kwargs (dict[str, Any])
use_init (bool)
container_strategy (Union[SearchStrategy[TypeVar(Example_co, covariant=True)], Callable[[type[TypeVar(Example_co, covariant=True)]], SearchStrategy[TypeVar(Example_co, covariant=True)]], None])
type_strategies (dict[type[object], Union[SearchStrategy[TypeVar(Example_co, covariant=True)], Callable[[type[TypeVar(Example_co, covariant=True)]], SearchStrategy[TypeVar(Example_co, covariant=True)]]]])

settings_kwargs: dict[str, Any]¶: Settings directly passed on to hypothesis. We support all kwargs from @settings, see @settings docs.

use_init: bool¶: Whether to create examples using __init__ instead of the default .

container_strategy: Union[SearchStrategy[TypeVar(Example_co, covariant=True)], Callable[[type[TypeVar(Example_co, covariant=True)]], SearchStrategy[TypeVar(Example_co, covariant=True)]], None]¶: Strategy for generating the container. By default, we generate examples of a container using: returns.contrib.hypothesis.containers.strategy_from_container().

type_strategies: dict[type[object], Union[SearchStrategy[TypeVar(Example_co, covariant=True)], Callable[[type[TypeVar(Example_co, covariant=True)]], SearchStrategy[TypeVar(Example_co, covariant=True)]]]]¶: Strategies for generating values of types other than the container and its lawful interfaces. This can be useful for overriding TypeVar, Callable, etc. in case you use certain types that hypothesis is unable to find.

default_settings(container_type)[source]¶

Return default settings for creating law tests.

We use some special strategies by default, but they can be overridden by the user if needed:

TypeVar: We need to make sure that the values generated behave sensibly when tested for equality.
collections.abc.Callable: We need to generate pure functions, which are not the default.

Note that this is collections.abc.Callable, NOT typing.Callable. This is because, at runtime, typing.get_origin(Callable[[int], str]) is collections.abc.Callable. So, this is the type we should register with hypothesis.

Parameters:: container_type (type[Lawful])
Return type:: Settings

check_all_laws(container_type, *, settings_kwargs=None, use_init=False, container_strategy=None, type_strategies=None)[source]¶

Function to check all defined mathematical laws in a specified container.

Should be used like so:

from returns.contrib.hypothesis.laws import check_all_laws
from returns.io import IO

check_all_laws(IO)

You can also pass different hypothesis settings inside:

check_all_laws(IO, settings_kwargs={'max_examples': 100})

Note

Cannot be used inside doctests because of the magic we use inside.

hypothesis plugin¶

Installation¶

hypothesis entrypoint¶

check_all_laws¶

Registering Custom Strategies when Checking Laws¶

Registering Custom Strategies outside Law Tests¶

Further reading¶

API Reference¶

Types we have already registered for you¶

DSL to register custom containers¶

DSL to define laws¶

Plugin internals¶