Static type checking support for Trio and related projects

async, trio, mypy
pip install trio-typing==0.7.0


Latest PyPI version Automated test status Code style: black Checked with mypy

trio-typing: static typing for Trio and related projects

This repository provides:

  • PEP 561 typing stubs packages for the Trio project packages:
  • A package trio_typing containing types that Trio programs often want to refer to (AsyncGenerator[Y, S] and TaskStatus[T]) (Nursery is exported publicly by mainline Trio as of version 0.12.0.)
  • A mypy plugin that smooths over some limitations in the basic type hints.

Supported platforms

Like Trio, we require Python 3.6 or later. Both PyPy and CPython are supported at runtime, but type checking must occur on CPython (due to limitations of mypy). We test on Linux using the latest releases from the 3.6, 3.7, and 3.8 branches, as well as 3.9-dev nightly. We're not knowingly doing anything OS-specific, so other OSes should work too.

Type checkers other than mypy are not supported, but might work. Experience reports and patches to add support are welcome.


runtime only

Install with:

pip install trio-typing

Then import some typing names from trio_typing, like TaskStatus; see below for more details.

with mypy support

Install trio-typing with mypy extras:

pip install trio-typing[mypy]

Note that due to recent plugin API changes, trio-typing 0.7.0+ requires mypy 0.920+.

Enable the plugin in your mypy.ini:

plugins = trio_typing.plugin

Start running mypy on your Trio code!

NOTE: trio-typing is the correct dependency to list in the requirements
of your own library using the trio_typing module. Don't use trio-typing[mypy], since that would needlessly add a mypy dependency to every app having a transitive dependency on your library.

What's in the box?

The stubs packages provide types for all public non-deprecated APIs of trio, outcome, and async_generator, as of the release date of the corresponding trio-typing distribution. You don't need to explicitly configure these; just say import trio (for example) and mypy will know to look in trio-stubs for the type information.

The trio_typing package provides:

  • TaskStatus[T], the type of the object passed as the task_status argument to a task started with nursery.start(). The type parameter T is the type of the value the task provides to be returned from nursery.start(). This is implemented as an ABC, and the actual private types inside Trio are registered as virtual subclasses of it. So, you can't instantiate trio_typing.TaskStatus, but isinstance(task_status, trio_typing.TaskStatus) where task_status is a Trio task status object does return True.
  • (Previous versions of trio_typing provided an analogous ABC for Nursery, but the actual class is available as trio.Nursery as of Trio 0.12.0; you should use that instead.)
  • A backport of typing.AsyncGenerator[YieldT, SendT] to Python 3.5. (YieldT is the type of values yielded by the generator, and SendT is the type of values it accepts as an argument to asend().) This is an abstract class describing the async generator interface: AsyncIterator plus asend, athrow, aclose, and the ag_* introspection attributes. On 3.6+, trio_typing.AsyncGenerator is just a reexport of typing.AsyncGenerator.
  • CompatAsyncGenerator[YieldT, SendT, ReturnT], a name for the otherwise-anonymous concrete async generator type returned by @async_generator functions. It is a subtype of AsyncGenerator[YieldT, SendT] and provides the same methods. (Native async generators don't have a ReturnT; it is only relevant in determining the return type of await async_generator.yield_from_().)
  • A few types that are only useful with the mypy plugin: YieldType[T], SendType[T], and the decorator @takes_callable_and_args.

The trio_typing.plugin mypy plugin provides:

  • Argument type checking for functions decorated with @asynccontextmanager (either the one in async_generator or the one in 3.7+ contextlib) and @async_generator

  • Inference of more specific trio.open_file() and return types based on constant mode and buffering arguments, so await trio.open_file("foo", "rb", 0) returns an unbuffered async file object in binary mode and await trio.open_file("bar") returns an async file object in text mode

  • Signature checking for task_status.started() with no arguments, so it raises an error if the task_status object is not of type TaskStatus[None]

  • Boilerplate reduction for functions that take parameters (fn, *args) and ultimately invoke fn(*args): just write:

    from mypy_extensions import VarArg
    def start_soon(
        async_fn: Callable[[VarArg()], Awaitable[T]],
        *args: Any,
        other_keywords: str = are_ok_too,
        # your implementation here

    start_soon(async_fn, *args) will raise an error if async_fn(*args) would do so. You can also make the callable take some non-splatted arguments; the *args get inserted at whatever position in the argument list you write VarArg().

    The above example will always fail when the plugin is not being used. If you want to always pass in such cases, you can use a union:

    def start_soon(
        async_fn: Union[
            Callable[..., Awaitable[T]],
            Callable[[VarArg()], Awaitable[T]],
        *args: Any,
        other_keywords: str = are_ok_too,
        # your implementation here

    Without the plugin, this type-checks fine (and allows inference of T), since any callable will match the Callable[..., Awaitable[T]] option. With the plugin, the entire union will be replaced with specific argument types.

    Note: due to mypy limitations, we only support a maximum of 5 positional arguments, and keyword arguments can't be passed in this way; nursery.start_soon(functools.partial(...)) will pass the type checker but won't be able to actually check the argument types.

  • Mostly-full support for type checking @async_generator functions. You write the decorated function as if it returned a union of its actual return type, its yield type wrapped in YieldType[], and its send type wrapped in SendType[]:

    from trio_typing import YieldType, SendType
    async def sleep_and_sqrt() -> Union[None, SendType[int], YieldType[float]]:
        next_yield = 0.0
        while True:
            amount = await yield_(next_yield)  # amount is an int
            if amount < 0:
                return None
            await trio.sleep(amount)
            next_yield = math.sqrt(amount)
    # prints: CompatAsyncGenerator[float, int, None]

    Calls to yield_ and yield_from_ inside an @async_generator function are type-checked based on these declarations. If you leave off either the yield type or send type, the missing one is assumed to be None; if you leave off both (writing just async def sleep_and_sqrt() -> None:, like you would if you weren't using the plugin), they're both assumed to be Any.

    Note the explicit return None; mypy won't accept return or falling off the end of the function, unless you run it with --no-warn-no-return.


  • Calls to variadic Trio functions like, nursery.start_soon(), and so on, only can type-check up to five positional arguments. (This number could be increased easily, but only at the cost of slower typechecking for everyone; mypy's current architecture requires that we generate overload sets initially for every arity we want to be able to use.) You can work around this with a # type: ignore comment.
  • outcome.capture() and outcome.acapture() currently don't typecheck their arguments at all.

Running the tests

trio-typing comes with a fairly extensive testsuite; it doesn't test all the mechanical parts of the stubs, but does exercise most of the interesting plugin behavior. You can run it after installing, with:

pytest -p trio_typing._tests.datadriven --pyargs trio_typing


Your choice of MIT or Apache 2.0.