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Atomic file writes.
.. code-block:: python
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.
API documentation <https://python-atomicwrites.readthedocs.io/en/latest/#api>_ for more
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
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 <https://social.msdn.microsoft.com/Forums/windowsdesktop/en-US/449bb49d-8acc-48dc-a46f-0760ceddbfc3/movefileexmovefilereplaceexisting-ntfs-same-volume-atomic?forum=windowssdk#a239bc26-eaf0-4920-9f21-440bd2be9cc8>
Doug Crook <https://social.msdn.microsoft.com/Profile/doug%20e.%20cook>, 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.
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
unlink. On Windows, it uses MoveFileEx_ through
ctypes with the appropriate flags.
Note that with
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
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 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.
is used, but there are no guarantees about disk internal buffers.
Atomicwrites is directly inspired by the following libraries (and shares a minimal amount of code):
The Trac project's
utility functions <http://www.edgewall.org/docs/tags-trac-0.11.7/epydoc/trac.util-pysrc.html>,
also used in
Werkzeug <http://werkzeug.pocoo.org/> and
mitsuhiko/python-atomicfile <https://github.com/mitsuhiko/python-atomicfile>_. The idea to use
ctypes instead of
PyWin32 originated there.
abarnert/fatomic <https://github.com/abarnert/fatomic>_. Windows support
PyWin32) was originally taken from there.
Other alternatives to atomicwrites include:
sashka/atomicfile <https://github.com/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
Boltons library collection <https://github.com/mahmoud/boltons>_
features a class for atomic file writes, which seems to have a very similar
overwrite parameter. It is lacking Windows support though.
Licensed under the MIT, see