safer 3.0.0

A safer file opener

✏️safer: a safer file opener ✏️

No more partial writes or corruption!

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

safer.open()

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 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 temporary file is moved over filename unless fp.safer_failed is true.

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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 the 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

NOTES

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 temporary file is moved over 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 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.

FUNCTIONS

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)

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

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

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

ARGUMENTS

make_parents:

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

delete_failures:

If true, the temporary file is deleted if there is an exception

follow_symlinks:

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

The remaining arguments are the same as for built-in open().