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.
See API documentation for more low-level interfaces.
Features that distinguish it from other similar libraries (see Alternatives and Credit):
Race-free assertion that the target file doesnât 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. Iâm basing my assumptions off a comment by Doug Crook, who appears to be a Microsoft employee:
Question: Is MoveFileEx atomic if the existing and new files are both on the same drive?
The simple answer is âusually, but in some cases it will silently fall-back to a non-atomic method, so donât count on itâ.
The implementation of MoveFileEx looks something like this: [â¦]
The problem is if the rename fails, you might end up with a CopyFile, which is definitely not atomic.
If you really need atomic-or-nothing, you can try calling NtSetInformationFile, which is unsupported but is much more likely to be atomic.
Simple high-level API that wraps a very flexible class-based API.
Consistent error handling across platforms.
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 stdlibâs ctypes with the appropriate flags.
Note that with link and unlink, thereâs 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 doesnât do it by itself.
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 disksâ internal buffers, but there donât 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):
ctypes instead of PyWin32 originated there.PyWin32) was originally taken from there.Other alternatives to atomicwrites include:
overwrite parameter. It is lacking Windows support though.Licensed under the MIT, see LICENSE.
atomicwrites.atomic_write(path, writer_cls=<class 'atomicwrites.AtomicWriter'>, **cls_kwargs)[source]¶
Simple atomic writes. This wraps AtomicWriter:
with atomic_write(path) as f:
f.write(...)
Parameters:
AtomicWriter to change some behavior and want to use that new subclass.Additional keyword arguments are passed to the writer class. See AtomicWriter.
All filesystem errors are subclasses of OSError.
ctypes are thrown.In either case, the errno attribute on the thrown exception maps to an errorcode in the errno module.
atomicwrites.replace_atomic(src, dst)[source]¶
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)[source]¶
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.
atomicwrites.AtomicWriter(path, mode='w', overwrite=False, **open_kwargs)[source]¶
A helper class for performing atomic writes. Usage:
with AtomicWriter(path).open() as f:
f.write(...)
Parameters:
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)[source]¶
Move the temporary file to the target location.
get_fileobject(suffix='', prefix='tmp', dir=None, **kwargs)[source]¶
Return the temporary file to use.
open()[source]¶
Open the temporary file.
rollback(f)[source]¶
Clean up all temporary resources.
sync(f)[source]¶
responsible for clearing as many file caches as possible before commit
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 âSoftwareâ), 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.
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