Atomic file writes.
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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.
See API documentation <https://python-atomicwrites.readthedocs.io/en/latest/#api>
_ 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 <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>
_
by Doug Cook <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
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.
.. _MoveFileEx: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365240%28v=vs.85%29.aspx
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 <https://msdn.microsoft.com/en-us/library/17618685.aspx>
_
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
(based on 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
subclassing).
The 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 LICENSE
.