Proper type annotation of Python functions with yield

Question

I figured out the answer on my own.

I searched, but found no documentation for the 3 type parameters of Generator in the official typing documentation for Python 3.5.2 – beyond a truly cryptic mention of…

class typing.Generator(Iterator[T_co], Generic[T_co, T_contra, V_co])

Luckily, the original PEP484 (that started all this) was far more helpful:

“The return type of generator functions can be annotated by the generic type Generator[yield_type, send_type, return_type] provided by typing.py module:

def echo_round() -> Generator[int, float, str]:
    res = yield
    while res:
        res = yield round(res)
    return 'OK'

Based on this, I was able to annotate my Generators, and saw mypy confirm my assignments:

from typing import Callable, Generator

# A protocol decoder:
#
# - yields Nothing
# - expects ints to be `send` in his yield waits
# - and doesn't return anything.
ProtocolDecodingCoroutine = Generator[None, int, None]

# A frame consumer (passed as an argument to a protocol decoder):
#
# - yields Nothing
# - expects List[int] to be `send` in his waiting yields
# - and doesn't return anything.
FrameConsumerCoroutine = Generator[None, List[int], None]


def unwrap_protocol(header: int=0x61,
                    footer: int=0x62,
                    dle :int=0xAB,
                    after_dle_func: Callable[[int], int]=lambda x: x,
                    target: FrameConsumerCoroutine=None) -> ProtocolDecodingCoroutine:
    ...

def frame_receiver() -> FrameConsumerCoroutine:
    ...

I tested my assignments by e.g. swaping the order of the types – and then as expected, mypy complained and asked for the proper ones (as seen above).

The complete code is accessible from here.

I will leave the question open for a couple of days, in case anyone wants to chime in – especially in terms of using the new coroutine styles of Python 3.5 (async def, etc) – I would appreciate a hint on exactly how they’d be used here.

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