mosquito / Aiofile
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AIOFile
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Real asynchronous file operations with asyncio support.
Status
Development - Stable
Features
- Since version 2.0.0 using
caio
_, is contain linux libaio and two thread-based implementations (c-based and pure-python). - AIOFile has no internal pointer. You should pass
offset
andchunk_size
for each operation or use helpers (Reader or Writer). The simples way is useasync_open
for create object with file-like interface. - For Linux using implementation based on
libaio
_. - For POSIX (MacOS X and optional Linux) using implementation
using on
threadpool
_. - Otherwise using pure-python thread-based implementation.
- Implementation chooses automatically depending on system compatibility.
.. _caio: https://pypi.org/project/caio .. _libaio: https://pagure.io/libaio .. _threadpool: https://github.com/mbrossard/threadpool/
Code examples
All code examples requires python 3.6+.
High-level API ++++++++++++++
async_open
helper
The ``async_open`` helper creates file like object with file-like methods:
.. code-block:: python
import asyncio
from aiofile import async_open
async def main():
async with async_open("/tmp/hello.txt", '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())
Supported methods:
* ``async def read(length = -1)`` - reading chunk from file, when length is
``-1`` will be read file to the end.
* ``async def write(data)`` - write chunk to file
* ``def seek(offset)`` - set file pointer position
* ``def tell()`` - returns current file pointer position
* ``async def readline(size=-1, newline="\n")`` - read chunks until
newline or EOF.
Suboptimal for small lines because doesn't reuse read buffer.
When you want to read file by lines please avoid to use ``async_open``
use ``LineReader`` instead.
``Reader`` and ``Writer``
When you want to read or write file linearly following example might be helpful.
.. code-block:: python
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.
.. code-block:: python
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.
Low-level API
+++++++++++++
Following API is just little bit sugared ``caio`` API.
Write and Read
~~~~~~~~~~~~~~
.. code-block:: python
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())
Read file line by line
~~~~~~~~~~~~~~~~~~~~~~
.. code-block:: python
import asyncio
from aiofile import AIOFile, LineReader, Writer
async def main():
async with AIOFile("/tmp/hello.txt", 'w') as afp:
writer = Writer(afp)
for i in range(10):
await writer("%d Hello World\n" % i)
await writer("Tail-less string")
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())
More examples
-------------
Useful examples with ``aiofile``
Async CSV Dict Reader
+++++++++++++++++++++
.. code-block:: python
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)
loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)
try:
loop.run_until_complete(main())
finally:
# Shutting down and closing file descriptors after interrupt
loop.run_until_complete(loop.shutdown_asyncgens())
loop.close()
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:
1. Upgrade the kernel
2. Use compatible file system
3. 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``.