Change file permissions symbolically (linux).

"""  module chmodsym.py (linux)

This module defines a function chmod(location, description)
which allows to change a file's permission with a symbolic
description of the mode, similar to the shell command 'chmod'.

Tested with python 2.6 and 3.1.
"""

import os, stat
import functools
import operator

def chmod(location, description):
    """chmod(location, description) --> None
    Change the access permissions of file, using a symbolic description
    of the mode, similar to the format of the shell command chmod.
    The format of description is
        * an optional letter in o, g, u, a (no letter means a)
        * an operator in +, -, =
        * a sequence of letters in r, w, x, or a single letter in o, g, u
    Example:
        chmod(myfile, "u+x")    # make the file executable for it's owner.
        chmod(myfile, "o-rwx")  # remove all permissions for all users not in the group. 
    See also the man page of chmod.
    """
    if chmod.regex is None:
        import re
        chmod.regex = re.compile(r"(?P<who>[uoga]?)(?P<op>[+\-=])(?P<value>[ugo]|[rwx]*)")
    mo = chmod.regex.match(description)
    who, op, value = mo.group("who"), mo.group("op"), mo.group("value")
    if not who:
        who = "a"
    mode = os.stat(location)[stat.ST_MODE]
    if value in ("o", "g", "u"):
        mask = ors((stat_bit(who, z) for z in "rwx" if (mode & stat_bit(value, z))))
    else:
        mask = ors((stat_bit(who, z) for z in value))
    if op == "=":
        mode &= ~ ors((stat_bit(who, z) for z in  "rwx"))
    mode = (mode & ~mask) if (op == "-") else (mode | mask)
    os.chmod(location, mode)

chmod.regex = None

# Helper functions

def stat_bit(who, letter):
    if who == "a":
        return stat_bit("o", letter) | stat_bit("g", letter) | stat_bit("u", letter)
    return getattr(stat, "S_I%s%s" % (letter.upper(), stat_bit.prefix[who]))

stat_bit.prefix = dict(u = "USR", g = "GRP", o = "OTH")

def ors(sequence, initial = 0):
    return functools.reduce(operator.__or__, sequence, initial)

# Test code

def test_code():

    import subprocess as sp

    def touch(location):
        sp.Popen("touch %s" % location, shell=True).wait()

    def get_mode_by_ls(location):
        """Run ls and return a string like '-r--r--r-' giving the mode."""
        popen = sp.Popen(["ls", "-l", location], stdout=sp.PIPE)
        sout, serr = popen.communicate()
        return sout[:10]

    loc = "xxxx.py"
    touch(loc)
    print(get_mode_by_ls(loc))
    for desc in ("g+rx", "a=g", "o-rw", "u=o", "+rwx", "a-wx"):
        chmod(loc, desc)
        print("%s --> %s" % (desc, get_mode_by_ls(loc)))

if __name__ == "__main__":
    test_code()