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mypy plugin¶We provide a custom mypy plugin to fix existing issues, provide new awesome features, and improve type-safety of things developers commonly use.
returns has [compatible-mypy] extra to install the supported version.
pip install 'returns[compatible-mypy]'
Or you can install mypy separately and check that version is supported.
To install our mypy plugin add it to the plugins section of the config file (setup.cfg or mypy.ini):
[mypy] plugins = returns.contrib.mypy.returns_plugin
Or in pyproject.toml:
[tool.mypy] plugins = ["returns.contrib.mypy.returns_plugin"]
We recommend to always add our plugin as the first one in chain.
Configuration¶You can have a look at the suggested mypy configuration in our own repository.
You can also use nitpick tool to enforce the same mypy configuration for all your projects.
We recommend to use our own setup. Add this to your pyproject.toml:
[tool.nitpick] style = "https://raw.githubusercontent.com/wemake-services/wemake-python-styleguide/master/styles/mypy.toml"
And use flake8 to lint that configuration defined in the setup matches yours. This will allow to keep them in sync with the upstream.
kind feature adds Higher Kinded Types (HKT) support
curry feature allows to write typed curried functions
partial feature allows to write typed partial application
flow feature allows to write better typed functional pipelines with flow function
pipe feature allows to write better typed functional pipelines with pipe function
do-notation feature allows using Do Notation
Used for typed partial function.
Used for typed curry decorator.
Used for typed flow call.
Used for typed pipe call.
Used for HKT emulation.
Used for Do Notation.
classDiagram
Plugin <|-- _ReturnsPlugin
Custom mypy plugin to solve the temporary problem with python typing.
Important: we don’t do anything ugly here. We only solve problems of the current typing implementation.
mypy API docs are here: https://mypy.readthedocs.io/en/latest/extending_mypy.html
We use pytest-mypy-plugins to test that it works correctly, see: https://github.com/mkurnikov/pytest-mypy-plugins
Plugin’s public API and entrypoint.
version (str)
type[Plugin]
Ensures that attribute access to KindN is correct.
In other words:
from typing import TypeVar
from returns.primitives.hkt import KindN
from returns.interfaces.mappable import MappableN
_MappableType = TypeVar('_MappableType', bound=MappableN)
kind: KindN[_MappableType, int, int, int]
reveal_type(kind.map) # will work correctly!
ctx (AttributeContext)
Type
Infers real type behind Kind form.
Basically, it turns Kind[IO, int] into IO[int]. The only limitation is that it works with only Instance type in the first type argument position.
So, dekind(KindN[T, int]) will fail.
ctx (FunctionContext)
Type
Returns the internal function wrapped as Kinded[def].
Works for Kinded class when __call__ magic method is used. See returns.primitives.hkt.Kinded for more information.
ctx (MethodSigContext)
CallableType
Reveals the correct return type of Kinded.__call__ method.
Turns -> KindN[I, t1, t2, t3] into -> I[t1, t2, t3].
Also strips unused type arguments for KindN, so: - KindN[IO, int, Never, Never] will be IO[int] - KindN[Result, int, str, Never] will be Result[int, str]
It also processes nested KindN with recursive strategy.
See returns.primitives.hkt.Kinded for more information.
ctx (MethodContext)
Type
Used to analyze @kinded method calls.
We do this due to __get__ descriptor magic.
ctx (MethodContext)
Type
Returns proper type for curried functions.
ctx (FunctionContext)
Type
This hook is used to make typed curring a thing in returns project.
This plugin is a temporary solution to the problem. It should be later replaced with the official way of doing things. One day functions will have better API and we plan to submit this plugin into mypy core plugins, so it would not be required.
Internally we just reduce the original function’s argument count. And drop some of them from function’s signature.
ctx (FunctionContext)
ProperType
Helps to analyze flow function calls.
By default, mypy cannot infer and check this function call:
>>> from returns.pipeline import flow >>> assert flow( ... 1, ... lambda x: x + 1, ... lambda y: y / 2, ... ) == 1.0
But, this plugin can! It knows all the types for all lambda functions in the pipeline. How?
We use the first passed parameter as the first argument to the first passed function
We use parameter + function to check the call and reveal types of current pipeline step
We iterate through all passed function and use previous return type as a new parameter to call current function
ctx (FunctionContext)
Type
Typing pipe functions requires several phases.
It is pretty obvious from its usage:
When we pass a sequence of functions we have to reduce the final callable type, it is require to match the callable protocol. And at this point we also kinda try to check that all pipeline functions do match, but this is impossible to do 100% correctly at this point, because generic functions don’t have a type argument to infer the final result
When we call the function, we need to check for two things. First, we check that passed argument fits our instance requirement. Second, we check that pipeline functions match. Now we have all arguments to do the real inference.
We also need to fix generic in method signature. It might be broken, because we add new generic arguments and return type. So, it is safe to reattach generic back to the function.
Here’s when it works:
>>> from returns.pipeline import pipe >>> def first(arg: int) -> bool: ... return arg > 0 >>> def second(arg: bool) -> str: ... return 'bigger' if arg else 'not bigger' >>> pipeline = pipe(first, second) # `analyzed` is called >>> assert pipeline(1) == 'bigger' # `signature and `infer` are called >>> assert pipeline(0) == 'not bigger' # `signature and `infer` again
This hook helps when we create the pipeline from sequence of funcs.
ctx (FunctionContext)
Type
This hook helps when we finally call the created pipeline.
ctx (MethodContext)
Type
Helps to fix generics in method signature.
ctx (MethodSigContext)
CallableType
Used to handle validation and error types in Do Notation.
What it does?
For all types we ensure that only a single container type is used in a single do-notation. We don’t allow mixing them.
For types with error types (like Result), it inferes what possible errors types can we have. The result is a Union of all possible errors.
Ensures that expression passed into .do method is literal.
Checks that default value is provided if generator expression has if conditions inside.
ctx (MethodContext)
Type
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