safer 4.0.0

A safer file opener

✏️safer: a safer file opener ✏️

No more partial writes or corruption! For file streams, sockets or any callable.

Install safer from the command line with pip (https://pypi.org/project/pip): pip install safer.

Tested on Python 3.4 and 3.8 For Python 2.7, use https://github.com/rec/safer/tree/v2.0.5

See the Medium article here.

safer is aimed at preventing a programmer error from causing corrupt files, streams, socket connections or similar. It does not prevent concurrent modification of files from other threads or processes: if you need atomic file writing, see https://pypi.org/project/atomicwrites/

• safer.writer() wraps an existing writer or socket and writes a whole response or nothing, by caching written data in memory

• safer.open() is a drop-in replacement for built-in open that writes a whole file or nothing by caching written data on disk. Unfortunately, disk caching does not work on Windows.

• safer.closer() returns a stream like from safer.write() that also closes the underlying stream or callable when it closes.

• safer.printer() is safer.open() except that it yields a a function that prints to the stream. Like safer.open(), it unfortunately does not work on Windows.

By default, safer buffers the written data in memory in a io.StringIO or io.BytesIO.

For very large files, safer.open() has a temp_file argument which writes the data to a temporary file on disk, which is moved over using os.rename if the operation completes successfully.

---

safer.writer()

safer.writer() wraps an existing stream - a writer, socket, or callback in a temporary stream which is only copied to the target stream at closer() and only if no exception was raised

EXAMPLE

sock = socket.socket(*args)

# dangerous
try:
    write_header(sock)
    write_body(sock)   # Exception is thrown here
    write_footer(sock)
 except:
    write_error(sock)  # Oops, the header was already written

# safer
try:
    with safer.writer(sock) as s:
        write_header(s)
        write_body(s)  # Exception is thrown here
        write_footer(s)
 except:
    write_error(sock)  # Nothing has been written

safer.open()

Writes a whole file or nothing. It’s a drop-in replacement for built-in open() except that safer.open() leaves the original file unchanged on failure.

EXAMPLE

# 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.

safer.open(filename) returns a file stream fp like open(filename) would, except that fp writes to memory stream or a a temporary file in the same directory.

If fp is used as a context manager and an exception is raised, then fp.safer_failed is automatically set to True. And when fp.close() is called, the cached data is stored in filename unless fp.safer_failed is true.

---

safer.printer()

safer.printer() is similar to safer.open() except it yields a function that prints to the open file - it’s very convenient for printing text.

Like safer.open(), if an exception is raised within its context manager, the original file is left unchanged.

EXAMPLE

# dangerous
with open(file, 'w') as fp:
    for item in items:
        print(item, file=fp)
    # Prints lines until the first exception

# safer
with safer.printer(file) as print:
    for item in items:
        print(item)
    # Either the whole file is written, or nothing

FUNCTIONS

safer.writer(stream, is_binary=None, close_on_exit=False, temp_file=False, chunk_size=1048576, delete_failures=True)

Write safely to file streams, sockets and callables.

safer.writer yields an in-memory stream that you can write to, but which is only written to the original stream if the context finished without raising an exception.

Because the actual writing happens when the context exits, it’s possible to block indefinitely if the underlying socket, stream or callable does.

ARGUMENTS

stream:

A file stream, a socket, or a callable that will receive data

is_binary:

Is stream a binary stream?

If is_binary is None, deduce whether it’s a binary file from the stream, or assume it’s text otherwise.

close_on_exit: If True, the underlying stream is closed when the writer

closes

temp_file:

If not false, use a disk file and os.rename() at the end, otherwise cache the writes in memory. If it’s a string, use this as the name of the temporary file, otherwise select one in the same directory as the target file, or in the system tempfile for streams that aren’t files.

chunk_size:

Transfer data from the temporary file to the underlying stream in chunks of this byte size

delete_failures:

If set to false, any temporary files created are not deleted if there is an exception

safer.open(name, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None, follow_symlinks=True, make_parents=False, delete_failures=True, temp_file=False)

A drop-in replacement for open() which returns a stream which only overwrites the original file when close() is called, and only if there was no failure.

If a stream fp return from safer.open() is used as a context manager and an exception is raised, the property fp.safer_failed is set to True.

In the method fp.close(), if fp.safer_failed is not set, then the cached results replace the original file, successfully completing the write.

If fp.safer_failed is true, then if delete_failures is true, the temporary file is deleted.

If the mode argument contains either 'a' (append), or '+' (update), then the original file will be copied to the temporary file before writing starts.

Note that if the temp_file argument is set, safer uses an extra temporary file which is renamed over the file only after the stream closes without failing. This uses as much disk space as the old and new files put together.

ARGUMENTS

The arguments mean the same as for built-in open(), except these:

follow_symlinks:

If true, overwrite the file pointed to and not the symlink

make_parents:

If true, create the parent directory of the file if it doesn’t exist

delete_failures:

If set to false, any temporary files created are not deleted if there is an exception

temp_file:

If true, use a disk file and os.rename() at the end, otherwise cache the writes in memory. If it’s a string, use this as the name of the temporary file, otherwise select one in the same directory as the target file, or in the system tempfile for streams that aren’t files.

safer.closer(stream, is_binary=None, close_on_exit=False)

Like safer.writer() but with close_on_exit=True by default

ARGUMENTS

stream:

A file stream, a socket, or a callable that will receive data

is_binary:

Is stream a binary stream?

If is_binary is None, deduce whether it’s a binary file from the stream, or assume it’s text otherwise.

close_on_exit: If True, the underlying stream is closed when the writer

closes

safer.printer(name, mode='w', *args, **kwargs)

A context manager that yields a function that prints to the opened file, only writing to the original file at the exit of the context, and only if there was no exception thrown

ARGUMENTS

Same as for safer.open()