Unix 'ls' permissions to octal

unix_perms.py

import re

ISAPERM = r'(^[ld\-](?:[r\-][w\-][x\-sS]){2}[r\-][w\-][x\-tT](?:\.)?$)'


def to_octal(perms):
    if re.compile(ISAPERM).match(perms):
        userperms = perms[1:4]
        groupperms = perms[4:7]
        otherperms = perms[7:10]

        numericaluserperms = int(b"".join(str(x) for x in map(lambda x:
                                 1 if x != '-' else 0, userperms)),
                                 2)
        numericalgroupperms = int(b"".join(str(x) for x in map(lambda x:
                                  1 if x != '-' else 0, groupperms)),
                                  2)
        numericalotherperms = int(b"".join(str(x) for x in map(lambda x:
                                  1 if x != '-' else 0, otherperms)),
                                  2)

        # Special permissions
        numericalspecialperms = 0

        if 's' in userperms.lower():
            numericalspecialperms += 4
            if 'S' in userperms:
                numericaluserperms -= 1
        if 's' in groupperms.lower():
            numericalspecialperms += 2
            if 'S' in groupperms:
                numericalgroupperms -= 1
        if 't' in otherperms.lower():
            numericalspecialperms += 1
            if 'T' in otherperms:
                numericalotherperms -= 1

        return "{}{}{}{}".format(numericalspecialperms, numericaluserperms,
                                 numericalgroupperms, numericalotherperms)

    else:
        return None