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I am using Python to write chunks of text to files in a single operation:
If the script is interrupted so a file write does not complete I want to have no file rather than a partially complete file. Can this be done?
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
The operation may fail on some Unix flavors if src and dst are on different filesystems.
It may not be atomic operation if src and dest locations are not on same filesystem
os.fsyncstep may be skipped if performance/responsiveness is more important than the data integrity in cases like power failure, system crash etc
A simple snippet that implements atomic writing using Python
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)
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:
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
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
with atomic_write('path/to/file') as f: f.write("allons-y!\n")
It’s based on this recipe.
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.
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()