Description:

aiofile 3.8.8

Real asynchronous file operations with asyncio support.

Status
Development - Stable


Features

Since version 2.0.0 using caio, which contains linux libaio and two
thread-based implementations (c-based and pure-python).
AIOFile has no internal pointer. You should pass offset and
chunk_size for each operation or use helpers (Reader or Writer).
The simples way is to use async_open for creating object with
file-like interface.
For Linux using implementation based on libaio.
For POSIX (MacOS X and optional Linux) using implementation
based on threadpool.
Otherwise using pure-python thread-based implementation.
Implementation chooses automatically depending on system compatibility.



Limitations

Linux native AIO implementation is not able to open special files.
Asynchronous operations against special fs like /proc/ /sys/ are not
supported by the kernel. It’s not a aiofile`s or `caio issue.
In this cases, you might switch to thread-based implementations
(see troubleshooting section).
However, when used on supported file systems, the linux implementation has a
smaller overhead and is preferred but it’s not a silver bullet.



Code examples
All code examples requires python 3.6+.

High-level API
async_open helper
Helper mimics python file-like objects, it returns file-like
objects with similar but async methods.
Supported methods:

async def read(length = -1) - reading chunk from file, when length is
-1, will be reading file to the end.
async def write(data) - writing chunk to file
def seek(offset) - setting file pointer position
def tell() - returns current file pointer position
async def readline(size=-1, newline="\n") - read chunks until
newline or EOF. Since version 3.7.0 __aiter__ returns LineReader.
This method is suboptimal for small lines because it doesn’t reuse read buffer.
When you want to read file by lines please avoid using async_open
use LineReader instead.

def __aiter__() -> LineReader - iterator over lines.
def iter_chunked(chunk_size: int = 32768) -> Reader - iterator over
chunks.
.file property contains AIOFile object

Basic example:
import asyncio
from pathlib import Path
from tempfile import gettempdir

from aiofile import async_open

tmp_filename = Path(gettempdir()) / "hello.txt"

async def main():
async with async_open(tmp_filename, 'w+') as afp:
await afp.write("Hello ")
await afp.write("world")
afp.seek(0)

print(await afp.read())

await afp.write("Hello from\nasync world")
print(await afp.readline())
print(await afp.readline())

loop = asyncio.get_event_loop()
loop.run_until_complete(main())
Example without context manager:
import asyncio
import atexit
import os
from tempfile import mktemp

from aiofile import async_open


TMP_NAME = mktemp()
atexit.register(os.unlink, TMP_NAME)


async def main():
afp = await async_open(TMP_NAME, "w")
await afp.write("Hello")
await afp.close()


asyncio.run(main())
assert open(TMP_NAME, "r").read() == "Hello"
Concatenate example program (cat):
import asyncio
import sys
from argparse import ArgumentParser
from pathlib import Path

from aiofile import async_open

parser = ArgumentParser(
description="Read files line by line using asynchronous io API"
)
parser.add_argument("file_name", nargs="+", type=Path)

async def main(arguments):
for src in arguments.file_name:
async with async_open(src, "r") as afp:
async for line in afp:
sys.stdout.write(line)


asyncio.run(main(parser.parse_args()))
Copy file example program (cp):
import asyncio
from argparse import ArgumentParser
from pathlib import Path

from aiofile import async_open

parser = ArgumentParser(
description="Copying files using asynchronous io API"
)
parser.add_argument("source", type=Path)
parser.add_argument("dest", type=Path)
parser.add_argument("--chunk-size", type=int, default=65535)


async def main(arguments):
async with async_open(arguments.source, "rb") as src, \
async_open(arguments.dest, "wb") as dest:
async for chunk in src.iter_chunked(arguments.chunk_size):
await dest.write(chunk)


asyncio.run(main(parser.parse_args()))
Example with opening already open file pointer:
import asyncio
from typing import IO, Any
from aiofile import async_open


async def main(fp: IO[Any]):
async with async_open(fp) as afp:
await afp.write("Hello from\nasync world")
print(await afp.readline())


with open("test.txt", "w+") as fp:
asyncio.run(main(fp))
Linux native aio doesn’t support reading and writing special files
(e.g. procfs/sysfs/unix pipes/etc.), so you can perform operations with
these files using compatible context objects.
import asyncio
from aiofile import async_open
from caio import thread_aio_asyncio
from contextlib import AsyncExitStack


async def main():
async with AsyncExitStack() as stack:

# Custom context should be reused
ctx = await stack.enter_async_context(
thread_aio_asyncio.AsyncioContext()
)

# Open special file with custom context
src = await stack.enter_async_context(
async_open("/proc/cpuinfo", "r", context=ctx)
)

# Open regular file with default context
dest = await stack.enter_async_context(
async_open("/tmp/cpuinfo", "w")
)

# Copying file content line by line
async for line in src:
await dest.write(line)


asyncio.run(main())


Low-level API
The AIOFile class 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 open file without moving the virtual carriage.
For example, you may make 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. Keep in mind
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 the built-in open.
import asyncio
from aiofile import AIOFile


async def main():
async with AIOFile("hello.txt", 'w+') as afp:
payload = "Hello world\n"

await asyncio.gather(
*[afp.write(payload, offset=i * len(payload)) for i in range(10)]
)

await afp.fsync()

assert await afp.read(len(payload) * 10) == payload * 10

asyncio.run(main())
The Low-level API in fact is just little bit sugared caio API.
import asyncio
from aiofile import AIOFile


async def main():
async with AIOFile("/tmp/hello.txt", 'w+') as afp:
await afp.write("Hello ")
await afp.write("world", offset=7)
await afp.fsync()

print(await afp.read())


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

Reader and Writer
When you want to read or write file linearly following example
might be helpful.
import asyncio
from aiofile import AIOFile, Reader, Writer


async def main():
async with AIOFile("/tmp/hello.txt", 'w+') as afp:
writer = Writer(afp)
reader = Reader(afp, chunk_size=8)

await writer("Hello")
await writer(" ")
await writer("World")
await afp.fsync()

async for chunk in reader:
print(chunk)


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


LineReader - read file line by line
LineReader is a helper that is very effective when you want to read a file
linearly and line by line.
It contains a buffer and will read the fragments of the file chunk by
chunk into the buffer, where it will try to find lines.
The default chunk size is 4KB.
import asyncio
from aiofile import AIOFile, LineReader, Writer


async def main():
async with AIOFile("/tmp/hello.txt", 'w+') as afp:
writer = Writer(afp)

await writer("Hello")
await writer(" ")
await writer("World")
await writer("\n")
await writer("\n")
await writer("From async world")
await afp.fsync()

async for line in LineReader(afp):
print(line)


loop = asyncio.get_event_loop()
loop.run_until_complete(main())
When you want to read file by lines please avoid to use async_open
use LineReader instead.




More examples
Useful examples with aiofile

Async CSV Dict Reader
import asyncio
import io
from csv import DictReader

from aiofile import AIOFile, LineReader


class AsyncDictReader:
def __init__(self, afp, **kwargs):
self.buffer = io.BytesIO()
self.file_reader = LineReader(
afp, line_sep=kwargs.pop('line_sep', '\n'),
chunk_size=kwargs.pop('chunk_size', 4096),
offset=kwargs.pop('offset', 0),
)
self.reader = DictReader(
io.TextIOWrapper(
self.buffer,
encoding=kwargs.pop('encoding', 'utf-8'),
errors=kwargs.pop('errors', 'replace'),
), **kwargs,
)
self.line_num = 0

def __aiter__(self):
return self

async def __anext__(self):
if self.line_num == 0:
header = await self.file_reader.readline()
self.buffer.write(header)

line = await self.file_reader.readline()

if not line:
raise StopAsyncIteration

self.buffer.write(line)
self.buffer.seek(0)

try:
result = next(self.reader)
except StopIteration as e:
raise StopAsyncIteration from e

self.buffer.seek(0)
self.buffer.truncate(0)
self.line_num = self.reader.line_num

return result


async def main():
async with AIOFile('sample.csv', 'rb') as afp:
async for item in AsyncDictReader(afp):
print(item)


asyncio.run(main())



Troubleshooting
The caio linux implementation works normal for modern linux kernel versions
and file systems. So you may have problems specific for your environment.
It’s not a bug and might be resolved some ways:

Upgrade the kernel
Use compatible file systems
Use threads based or pure python implementation.

The caio since version 0.7.0 contains some ways to do this.
1. In runtime use the environment variable CAIO_IMPL with
possible values:

linux - use native linux kernels aio mechanism
thread - use thread based implementation written in C
python - use pure python implementation

2. File default_implementation located near __init__.py in caio
installation path. It’s useful for distros package maintainers. This file
might contains comments (lines starts with # symbol) and the first line
should be one of linux thread or python.

You might manually manage contexts:

import asyncio

from aiofile import async_open
from caio import linux_aio_asyncio, thread_aio_asyncio


async def main():
linux_ctx = linux_aio_asyncio.AsyncioContext()
threads_ctx = thread_aio_asyncio.AsyncioContext()

async with async_open("/tmp/test.txt", "w", context=linux_ctx) as afp:
await afp.write("Hello")

async with async_open("/tmp/test.txt", "r", context=threads_ctx) as afp:
print(await afp.read())


asyncio.run(main())

License

For personal and professional use. You cannot resell or redistribute these repositories in their original state.

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