python-atomicwrites¶
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 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. I’m 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 “usually, but in some cases it will silently fall-back to a non-atomic method, so don’t count on it”.
The implementation of MoveFileEx looks something like this: […]
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.
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 ofPyWin32
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)[source]¶ 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)[source]¶ Move
src
todst
. Ifdst
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
todst
. There might a timewindow where both filesystem entries exist. Ifdst
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, **open_kwargs)[source]¶ 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. This defaults to wb in Python 2 and w in Python 3.
- 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.
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.