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===================
python-atomicwrites
===================
:alt: Documentation Status
**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] 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 Cook
], 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.
.. _MoveFileEx:
https://msdn.microsoft.com/en-us/library/windows/desktop/aa365240%28v=vs.85%29.aspx
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
=======================
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