How to make file creation an atomic operation?

Solution 1:

Write data to a temporary file and when data has been successfully written, rename the file to the correct destination file e.g

f = open(tmpFile, 'w')
f.write(text)
# make sure that all data is on disk
# see http://stackoverflow.com/questions/7433057/is-rename-without-fsync-safe
f.flush()
os.fsync(f.fileno()) 
f.close()

os.rename(tmpFile, myFile)

According to doc http://docs.python.org/library/os.html#os.rename

If successful, the renaming will be an atomic operation (this is a POSIX requirement). On Windows, if dst already exists, OSError will be raised even if it is a file; there may be no way to implement an atomic rename when dst names an existing file

also

The operation may fail on some Unix flavors if src and dst are on different filesystems.

Note:

• It may not be atomic operation if src and dest locations are not on same filesystem

• os.fsync step may be skipped if performance/responsiveness is more important than the data integrity in cases like power failure, system crash etc

Solution 2:

A simple snippet that implements atomic writing using Python tempfile.

with open_atomic('test.txt', 'w') as f:
    f.write("huzza")

or even reading and writing to and from the same file:

with open('test.txt', 'r') as src:
    with open_atomic('test.txt', 'w') as dst:
        for line in src:
            dst.write(line)

using two simple context managers

import os
import tempfile as tmp
from contextlib import contextmanager

@contextmanager
def tempfile(suffix='', dir=None):
    """ Context for temporary file.

    Will find a free temporary filename upon entering
    and will try to delete the file on leaving, even in case of an exception.

    Parameters
    ----------
    suffix : string
        optional file suffix
    dir : string
        optional directory to save temporary file in
    """

    tf = tmp.NamedTemporaryFile(delete=False, suffix=suffix, dir=dir)
    tf.file.close()
    try:
        yield tf.name
    finally:
        try:
            os.remove(tf.name)
        except OSError as e:
            if e.errno == 2:
                pass
            else:
                raise

@contextmanager
def open_atomic(filepath, *args, **kwargs):
    """ Open temporary file object that atomically moves to destination upon
    exiting.

    Allows reading and writing to and from the same filename.

    The file will not be moved to destination in case of an exception.

    Parameters
    ----------
    filepath : string
        the file path to be opened
    fsync : bool
        whether to force write the file to disk
    *args : mixed
        Any valid arguments for :code:`open`
    **kwargs : mixed
        Any valid keyword arguments for :code:`open`
    """
    fsync = kwargs.get('fsync', False)

    with tempfile(dir=os.path.dirname(os.path.abspath(filepath))) as tmppath:
        with open(tmppath, *args, **kwargs) as file:
            try:
                yield file
            finally:
                if fsync:
                    file.flush()
                    os.fsync(file.fileno())
        os.rename(tmppath, filepath)

Solution 3:

Since it is very easy to mess up with the details, I recommend using a tiny library for that. The advantage of a library is that it takes care all these nitty-gritty details, and is being reviewed and improved by a community.

One such library is python-atomicwrites by untitaker which even has proper Windows support:

• https://github.com/untitaker/python-atomicwrites
• https://pypi.org/project/atomicwrites/

From the README:

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.

Installation via PIP:

pip install atomicwrites

Solution 4:

I’m using this code to atomically replace/write a file:

import os
from contextlib import contextmanager

@contextmanager
def atomic_write(filepath, binary=False, fsync=False):
    """ Writeable file object that atomically updates a file (using a temporary file).

    :param filepath: the file path to be opened
    :param binary: whether to open the file in a binary mode instead of textual
    :param fsync: whether to force write the file to disk
    """

    tmppath = filepath + '~'
    while os.path.isfile(tmppath):
        tmppath += '~'
    try:
        with open(tmppath, 'wb' if binary else 'w') as file:
            yield file
            if fsync:
                file.flush()
                os.fsync(file.fileno())
        os.rename(tmppath, filepath)
    finally:
        try:
            os.remove(tmppath)
        except (IOError, OSError):
            pass

Usage:

with atomic_write('path/to/file') as f:
    f.write("allons-y!\n")

It’s based on this recipe.

Solution 5:

Answers on this page are quite old, there are now libraries that do this for you.

In particular safer is a library designed to help prevent programmer error from corrupting files, socket connections, or generalized streams. It's quite flexible and amongst other things it has the option to use either memory or temporary files, you can even keep the temp files in case of failure.

Their example is just what you want:

# dangerous
with open(filename, 'w') as fp:
    json.dump(data, fp)
    # If an exception is raised, the file is empty or partly written

# safer
with safer.open(filename, 'w') as fp:
    json.dump(data, fp)
    # If an exception is raised, the file is unchanged.

It's in PyPI, just install it using pip install --user safer or get the latest at https://github.com/rec/safer