jwodder/inplace

In-place file processing in Python

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The in_place module provides an InPlace class for reading & writing a file "in-place": data that you write ends up at the same filepath that you read from, and in_place takes care of all the necessary mucking about with temporary files for you.

For example, given the file somefile.txt:

'Twas brillig, and the slithy toves
    Did gyre and gimble in the wabe;
All mimsy were the borogoves,
    And the mome raths outgrabe.

and the program disemvowel.py:

import in_place

with in_place.InPlace("somefile.txt") as fp:
    for line in fp:
        fp.write("".join(c for c in line if c not in "AEIOUaeiou"))

after running the program, somefile.txt will have been edited in place, reducing it to just:

'Tws brllg, nd th slthy tvs
    Dd gyr nd gmbl n th wb;
ll mmsy wr th brgvs,
    nd th mm rths tgrb.

and no sign of those pesky vowels remains! If you want a sign of those pesky vowels to remain, you can instead save the file's original contents in, say, somefile.txt~ by constructing the filehandle with:

in_place.InPlace("somefile.txt", backup_ext="~")

or save to someotherfile.txt with:

in_place.InPlace("somefile.txt", backup="someotherfile.txt")

Compared to the in-place filtering implemented by the Python standard library's fileinput module, in_place offers the following benefits:

• Instead of hijacking sys.stdout, a new filehandle is returned for writing.
• The filehandle supports all of the standard I/O methods, not just readline().
• There are options for setting the encoding, encoding error handling, and newline policy for opening the file, along with support for opening files in binary mode, and these options apply to both input and output.
• The complete filename of the backup file can be specified; you aren't constrained to just adding an extension.
• When used as a context manager, in_place will restore the original file if an exception occurs.
• The creation of temporary files won't silently clobber innocent bystander files.

Installation

in_place requires Python 3.8 or higher. Just use pip for Python 3 (You have pip, right?) to install it:

python3 -m pip install in_place

Basic Usage

in_place provides a single class, InPlace. Its constructor takes the following arguments:

name=<PATH> (required)

The path to the file to open & edit in-place

mode=<"b"|"t"|None>

Whether to operate on the file in binary or text mode. If mode is "b", the file will be opened in binary mode, and data will be read & written as bytes objects. If mode is "t" or None (the default), the file will be opened in text mode, and data will be read & written as str objects.

backup=<PATH>

If set, the original contents of the file will be saved to the given path when the instance is closed. backup cannot be set to the empty string.

backup_ext=<EXTENSION>

If set, the path to the backup file will be created by appending backup_ext to the original file path.

backup and backup_ext are mutually exclusive. backup_ext cannot be set to the empty string.

**kwargs

Any additional keyword arguments (such as encoding, errors, and newline) will be forwarded to open() when opening both the input and output file streams.

name, backup, and backup_ext can be str, filesystem-encoded bytes, or path-like objects.

InPlace instances act as read-write filehandles with the usual filehandle attributes, specifically:

__iter__()              __next__()              closed
flush()                 name                    read()
read1() *               readinto() *            readinto1() *
readline()              readlines()             write()
writelines()

* binary mode only

InPlace instances also feature the following new or modified attributes:

close()

Close filehandles and move files to their final destinations. If called after the filehandle has already been closed, close() does nothing.

Be sure to always close your instances when you're done with them by calling close() or rollback() either explicitly or implicitly (i.e., via use as a context manager).

rollback()

Like close(), but discard the output data (keeping the original file intact) instead of replacing the original file with it

__enter__(), __exit__()

When an InPlace instance is used as a context manager, on exiting the context, the instance will be either closed (if all went well) or rolled back (if an exception occurred). InPlace context managers are not reusable but are reentrant (as long as no further operations are performed after the innermost context ends).

input

The actual filehandle that data is read from, in case you need to access it directly

output

The actual filehandle that data is written to, in case you need to access it directly