AIOFile context manager looses data

[termim]

When AIOFile object is used as a context manager then some data is lost. Below is slightly modified example from README.rst:

import asyncio
from aiofile import AIOFile, LineReader, async_open


async def main():

    print("\nAIOFile:")
    async with AIOFile("/tmp/hello.txt", 'w') as afp:

        for i in range(10):
            await afp.write("%d Hello World\n" % i)

        await afp.write("Tail-less string\n")

    async with AIOFile("/tmp/hello.txt", 'r') as afp:
        async for line in LineReader(afp):
            print(line[:-1])

    print("\nasync_open:")
    async with async_open("/tmp/hello.txt", 'w') as afp:

        for i in range(10):
            await afp.write("%d Hello World\n" % i)

        await afp.write("Tail-less string\n")

    async with AIOFile("/tmp/hello.txt", 'r') as afp:
        async for line in LineReader(afp):
            print(line[:-1])


loop = asyncio.get_event_loop()
loop.run_until_complete(main())

The output is:

Python 3.8.16 (default, Dec 10 2022, 12:00:00) 
[GCC 7.5.0] on localhost
>>> 
AIOFile:
Tail-less string

async_open:
0 Hello World
1 Hello World
2 Hello World
3 Hello World
4 Hello World
5 Hello World
6 Hello World
7 Hello World
8 Hello World
9 Hello World
Tail-less string
>>> 

So it seems that async_open worked fine while AIOFile context manager lost what was written in the for loop.

[mosquito]

This code works as it should. The fact is that AIOFile is a low-level interface for asynchronous file operations, and the read and write methods accept an offset=0 in bytes at which the operation will be performed.

This allows you to do many independent IO operations on an once opened file without moving the virtual carriage.

For example, you may made 10 concurrent HTTP requests by specifying the Range header, and asynchronously write one opened file, while the offsets must either be calculated manually, or use 10 instances of Writer with specified initial offsets.

In order to provide sequential reading and writing, there is Writer, Reader and LineReader. async_open is not the same as AIOFile, it provides a similar interface for file operations, it simulates methods like read or write as it is implemented in a built-in open.

[termim]

Thank you for the explanation!

[mosquito]

@termim added this explanation to https://github.com/mosquito/aiofile#low-level-api