How to make file creation an atomic operation?

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

Just link the file after you're done:

with tempfile.NamedTemporaryFile(mode="w") as f:
    f.write(...)
    os.link(f.name, final_filename)

If you want to get fancy:

@contextlib.contextmanager
def open_write_atomic(filename: str, **kwargs):
    kwargs['mode'] = 'w'
    with tempfile.NamedTemporaryFile(**kwargs) as f:
        yield f
        os.link(f.name, filename)

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

Atomic solution for Windows to loop folder and rename files. Tested, atomic to automate, you can increase probability to minimize risk not to event of having same file name. You random library for letter symbols combinations use random.choice method, for digit str(random.random.range(50,999999999,2). You can vary digits range as you want.

import os import random

path = "C:\\Users\\ANTRAS\\Desktop\\NUOTRAUKA\\"

def renamefiles():
    files = os.listdir(path)
    i = 1
    for file in files:
        os.rename(os.path.join(path, file), os.path.join(path, 
                  random.choice('ABCDEFGHIJKL') + str(i) + str(random.randrange(31,9999999,2)) + '.jpg'))
        i = i+1

for x in range(30):
    renamefiles()