Very poor attempt at a Bank Street Writer (IBM PC) file decoder

bsw_decode.py

# Despite what's written about the Apple II version, the PC version of Bank Street Writer
# is a basic CGA word processor that uses ASCII-encoded files with some control codes.
# Files are stored without an extension, any files with an extension are password-protected.
# The extension is an encoded version of the password. Passwords are limited to 3 characters as a result.

import sys

control_codes = {
    243: '{pb}',
    244: '<c>',
    245: '\t',
    246: '{m}',
    247: '<u>',
    248: '<b>',
    249: '</u>',
    250: '</b>'
}

f = open(sys.argv[1], 'rb').read()
res = ''

for i in f:
    if 31 < i < 0x80:
        res += chr(i)
    elif i == 0xD:
        res += '\n'
    elif i in control_codes:
        res += control_codes[i]
    else:
        res += f'%{i:02X}%'

print(res)