surprises from get_standard_streams

[dougransom]

I was expecting the streams from get_standard_streams to provide awaitables when performing i/o. That doesn't seem to be the case.

I don't understand the purpose of get_standard_streams at all.

>>> i,o=await con.get_standard_streams() 
>>> dir(o)
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'buffer', 'close', 'drain', 'is_closing', 'loop', 'stream', 'task_finalizer', 'wait_closed', 'write', 'write_task']
>>> o.write("one")
one>>> o.write("two")
two>>> await o.write("Three")
Traceback (most recent call last):
  File "C:\Users\dougr\OneDrive\doug\codingprojects\aioutil\env\lib\site-packages\aioconsole\execute.py", line 140, in aexec
    result, new_local = await coro
  File "<console>", line 2, in __corofn
TypeError: object NoneType can't be used in 'await' expression

[vxgmichel]

I was expecting the streams from get_standard_streams to provide awaitables when performing i/o. That doesn't seem to be the case.

The objects returned by get_standard_streams are instances of StreamReader and StreamWriter, so the write method is indeed synchronous. To the contrary, the read, readline and drain methods are asynchronous.

I don't understand the purpose of get_standard_streams at all.

It's more of an internal helper to get access to an asyncio-compatible interface for stdin and stdout.

I hope that helps.

[dougransom]

Well I was looking for a asyncio-compatible interface for stdin, get you can see the standard output stream write operation from get_standard_streams is not awaitable:

two>>> await o.write("Three") Traceback (most recent call last): File "C:\Users\dougr\OneDrive\doug\codingprojects\aioutil\env\lib\site-packages\aioconsole\execute.py", line 140, in aexec result, new_local = await coro File "<console>", line 2, in __corofn TypeError: object NoneType can't be used in 'await' expression

So this is or isn't compatible with asyncio?

[vxgmichel]

So this is or isn't compatible with asyncio?

It is. In the asyncio world, writing to a stream is usually synchronous as you can see in the StreamWriter interface. It's the same if you open a tcp connection using asyncio.open_connection. See this exemple from the documentation:

import asyncio

async def tcp_echo_client(message):
    reader, writer = await asyncio.open_connection(
        '127.0.0.1', 8888)

    print(f'Send: {message!r}')
    writer.write(message.encode())
    await writer.drain()

    data = await reader.read(100)
    print(f'Received: {data.decode()!r}')

    print('Close the connection')
    writer.close()
    await writer.wait_closed()

asyncio.run(tcp_echo_client('Hello World!'))

Also note the use of StreamWriter.drain

[dougransom]

Thanks for the explanation, i was incorrectly expecting the same behavior as aoifiles write interface and i didn't get the drain method.

[vxgmichel]

Alright :)

[dougransom]

if aioconsole provides an interface to stdin, presumably it would be possible to adapt to general files?

[vxgmichel]

I'm not sure I understand your point, could you elaborate?

[dougransom]

Well something like this using file IO.
with something.open("myfile.txt") as writer:
writer.write("some text") await writer.drain()

which aoiconsole can already do with stdout.

presumably writer.drain() would complete once the data is written to the file?

[vxgmichel]

Isn't it what aiofiles is doing already? I think it's fine to use both aioconsole and aiofiles in the same project.

[dougransom]

aiofiles uses an asynchronous write interface:

async with aiofiles.tempfile.TemporaryFile('wb') as f: await f.write(b'Hello, World!')