Function to write the contents, suitably encoded, of a text file, e.g., a Python source file, into an extended attribute of a target file

ArchiveSourceWithOutput.py

# ArchiveSourceWithOutput.py
#
#   Copyright 2017 by David Lawrence Goldsmith
#   License: BSD
#   Use at own risk; no warranty expressed or implied
#
# Two functions--encode_and_write and read_and_decode--to assist in,
# among other potential uses, matching source code with output files,
# (on platforms that support extended attributes via the xattr shell
# command, e.g., Mac and some posix flavors):
#
# encode_and_write(source_file, target_file, xattr_name='source')
#   converts (the ascii content of) <source_file> (a valid path, as a
#   str) into a valid shell command line argument value, (i.e., a double-
#   quoted string free of other double-quotes, standard parentheses, and
#   newline characters), and "attaches" the result to <target_file> as
#   extended attribute <xattr_name> (default value 'source').
#
#   Example use-case: storage of the exact code that produces some output
#   file(s) as part of those output file(s), regardless of output format,
#   i.e., text source can be "pinned" to image files, pdf's, even directories
#   and sym. links, etc.
#
# read_and_decode(target_file, xattr_name='source')
#   encode_and_write's "inverse": reads in <target_file>'s extended attribute
#   <xattr_name> value, and converts it back into the original text file,
#   writing it out to a file with the same full path as the original, but with a '_'
#   preceding any '.' (TODO: add support for alternate output name in read_and_decode)
#
# Example usage:
#
# from ArchiveSourceWithOutput import *
#         :
# msg = encode_and_write(__file__, targetfilepath)
# if msg: print(msg) # msg is empty string unless something goes wrong
# else:
#     msg = read_and_decode(targetfilepath) # Ditto
#     print(msg)

from os import system, linesep
from subprocess import getoutput

def encode_and_write(source_file, target_file, xattr_name='source'):
    fail = getoutput('xattr')
    if fail:
        pass
    else:
        src = open(source_file)
        temp = src.readlines()
        src.close()
        cnts = source_file + ' _fn_ '
        for line in temp:
            cnts += line[:-1] + ' _nl_ '
        cnts = cnts.replace('(', '_lparen_')
        cnts = cnts.replace(')', '_rparen_')
        cnts = cnts.replace('"', '_dblquote_')

        fail = getoutput('chmod 660 ' + target_file)
        if fail:
            pass
        else:
            fail = getoutput('xattr -w ' + xattr_name + ' "' + cnts + '" ' + target_file)

    return fail

def read_and_decode(target_file, xattr_name='source'):
    fail = system('xattr -p ' + xattr_name + ' ' + target_file)
    if fail:
        return fail
    else:
        fail = getoutput('xattr -p ' + xattr_name + ' ' + target_file)
        filename, fail = fail.split(' _fn_ ', 1)
        filename = filename.replace('.', '_.')
        fail = fail.replace('_dblquote_', '"')
        fail = fail.replace('_rparen_', ')')
        fail = fail.replace('_lparen_', '(')
        fail = fail.replace(' _nl_ ', linesep)
        fl = open(filename, 'w')
        fl.write(fail)
        fl.close()
        return ''