c02bd11b93
This is a first stab at solving https://github.com/astral-sh/ty/issues/500, at least in part, with the old solver. We add a new `TypeRelation` that lets us opt into using constraint sets to describe when a typevar is assignability to some type, and then use that to calculate a constraint set that describes when two callable types are assignable. If the callable types contain typevars, that constraint set will describe their valid specializations. We can then walk through all of the ways the constraint set can be satisfied, and record a type mapping in the old solver for each one. --------- Co-authored-by: Carl Meyer <carl@astral.sh> Co-authored-by: Alex Waygood <alex.waygood@gmail.com>
120 lines
2.2 KiB
Markdown
120 lines
2.2 KiB
Markdown
# `async` / `await`
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## Basic
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```py
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async def retrieve() -> int:
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return 42
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async def main():
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result = await retrieve()
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reveal_type(result) # revealed: int
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```
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## Generic `async` functions
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```py
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from typing import TypeVar
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T = TypeVar("T")
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async def persist(x: T) -> T:
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return x
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async def f(x: int):
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result = await persist(x)
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reveal_type(result) # revealed: int
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```
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## Use cases
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### `Future`
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```py
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import asyncio
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import concurrent.futures
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def blocking_function() -> int:
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return 42
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async def main():
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loop = asyncio.get_event_loop()
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with concurrent.futures.ThreadPoolExecutor() as pool:
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result = await loop.run_in_executor(pool, blocking_function)
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reveal_type(result) # revealed: int
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```
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### `asyncio.Task`
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```py
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import asyncio
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async def f() -> int:
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return 1
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async def main():
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task = asyncio.create_task(f())
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result = await task
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reveal_type(result) # revealed: int
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```
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### `asyncio.gather`
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```py
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import asyncio
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async def task(name: str) -> int:
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return len(name)
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async def main():
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(a, b) = await asyncio.gather(
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task("A"),
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task("B"),
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)
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reveal_type(a) # revealed: int
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reveal_type(b) # revealed: int
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```
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## Under the hood
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```toml
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[environment]
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python-version = "3.12" # Use 3.12 to be able to use PEP 695 generics
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```
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Let's look at the example from the beginning again:
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```py
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async def retrieve() -> int:
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return 42
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```
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When we look at the signature of this function, we see that it actually returns a `CoroutineType`:
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```py
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reveal_type(retrieve) # revealed: def retrieve() -> CoroutineType[Any, Any, int]
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```
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The expression `await retrieve()` desugars into a call to the `__await__` dunder method on the
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`CoroutineType` object, followed by a `yield from`. Let's first see the return type of `__await__`:
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```py
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reveal_type(retrieve().__await__()) # revealed: Generator[Any, None, int]
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```
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We can see that this returns a `Generator` that yields `Any`, and eventually returns `int`. For the
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final type of the `await` expression, we retrieve that third argument of the `Generator` type:
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```py
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from typing import Generator
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def _():
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result = yield from retrieve().__await__()
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reveal_type(result) # revealed: int
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```
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