atomicwrites man page

atomicwrites — atomicwrites Documentation

Atomic file writes.

from atomicwrites import atomic_write

with atomic_write('foo.txt', overwrite=True) as f:
    f.write('Hello world.')
    # "foo.txt" doesn't exist yet.

# Now it does.

Features that distinguish it from other similar libraries (see Alternatives and Credit):

·
Race-free assertion that the target file doesn't yet exist. This can be controlled with the overwrite parameter.
·
Windows support, although not well-tested. The MSDN resources are not very explicit about which operations are atomic.
·
Simple high-level API that wraps a very flexible class-based API.
·
Consistent error handling across platforms.

How It Works

It uses a temporary file in the same directory as the given path. This ensures that the temporary file resides on the same filesystem.

The temporary file will then be atomically moved to the target location: On POSIX, it will use rename if files should be overwritten, otherwise a combination of link and unlink. On Windows, it uses MoveFileEx through stdlib's ctypes with the appropriate flags.

Note that with link and unlink, there's a timewindow where the file might be available under two entries in the filesystem: The name of the temporary file, and the name of the target file.

Also note that the permissions of the target file may change this way. In some situations a chmod can be issued without any concurrency problems, but since that is not always the case, this library doesn't do it by itself.

fsync

On POSIX, fsync is invoked on the temporary file after it is written (to flush file content and metadata), and on the parent directory after the file is moved (to flush filename).

fsync does not take care of disks' internal buffers, but there don't seem to be any standard POSIX APIs for that. On OS X, fcntl is used with F_FULLFSYNC instead of fsync for that reason.

On Windows, _commit is used, but there are no guarantees about disk internal buffers.

Alternatives and Credit

Atomicwrites is directly inspired by the following libraries (and shares a minimal amount of code):

·
The Trac project's utility functions, also used in Werkzeug and mitsuhiko/python-atomicfile. The idea to use ctypes instead of PyWin32 originated there.
·
abarnert/fatomic. Windows support (based on PyWin32) was originally taken from there.

Other alternatives to atomicwrites include:

·
sashka/atomicfile. Originally I considered using that, but at the time it was lacking a lot of features I needed (Windows support, overwrite-parameter, overriding behavior through subclassing).
·
The Boltons library collection features a class for atomic file writes, which seems to have a very similar overwrite parameter. It is lacking Windows support though.

License

Licensed under the MIT, see License.

API

atomicwrites.atomic_write(path, writer_cls=<class 'atomicwrites.AtomicWriter'>, **cls_kwargs)

Simple atomic writes. This wraps AtomicWriter:

with atomic_write(path) as f:
    f.write(...)
Parameters
·
path -- The target path to write to.
·
writer_cls -- The writer class to use. This parameter is useful if you subclassed AtomicWriter to change some behavior and want to use that new subclass.

Additional keyword arguments are passed to the writer class. See AtomicWriter.

Errorhandling

All filesystem errors are subclasses of OSError.

·
On UNIX systems, errors from the Python stdlib calls are thrown.
·
On Windows systems, errors from Python's ctypes are thrown.

In either case, the errno attribute on the thrown exception maps to an errorcode in the errno module.

Low-level API

atomicwrites.replace_atomic(src, dst)
Move src to dst. If dst exists, it will be silently overwritten.

Both paths must reside on the same filesystem for the operation to be atomic.
atomicwrites.move_atomic(src, dst)
Move src to dst. There might a timewindow where both filesystem entries exist. If dst already exists, FileExistsError will be raised.

Both paths must reside on the same filesystem for the operation to be atomic.
class atomicwrites.AtomicWriter(path, mode='w', overwrite=False)

A helper class for performing atomic writes. Usage:

with AtomicWriter(path).open() as f:
    f.write(...)
Parameters
·
path -- The destination filepath. May or may not exist.
·
mode -- The filemode for the temporary file.
·
overwrite -- If set to false, an error is raised if path exists. Errors are only raised after the file has been written to. Either way, the operation is atomic.

If you need further control over the exact behavior, you are encouraged to subclass.

commit(f)
Move the temporary file to the target location.
get_fileobject(dir=None, **kwargs)
Return the temporary file to use.
open()
Open the temporary file.
rollback(f)
Clean up all temporary resources.
sync(f)
responsible for clearing as many file caches as possible before commit

License

Copyright (c) 2015-2016 Markus Unterwaditzer

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR Copyright HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Author

Markus Unterwaditzer

Info

Sep 04, 2016 1.1 atomicwrites