Generates cyclic pattern

pattern.py

#!/usr/bin/env python3

import sys
from string import ascii_uppercase, ascii_lowercase, digits

MAX_PATTERN_LENGTH = 20280

class MaxLengthException(Exception):
    pass

class WasNotFoundException(Exception):
    pass


def pattern_gen(length):
    """
    Generate a pattern of a given length up to a maximum
    of 20280 - after this the pattern would repeat
    """
    if length >= MAX_PATTERN_LENGTH:
        raise MaxLengthException('ERROR: Pattern length exceeds maximum of %d' % MAX_PATTERN_LENGTH)

    pattern = ''
    for upper in ascii_uppercase:
        for lower in ascii_lowercase:
            for digit in digits:
                if len(pattern) < length:
                    pattern += upper+lower+digit
                else:
                    out = pattern[:length]
                    return out


def pattern_search(search_pattern):
    """
    Search for search_pattern in pattern.  Convert from hex if needed
    Looking for needle in haystack
    """
    needle = search_pattern

    try:
        if needle.startswith('0x'):
            # Strip off '0x', convert to ASCII and reverse
            needle = needle[2:]
            needle = bytes.fromhex(needle).decode('ascii')
            needle = needle[::-1]
    except TypeError as e:
        print('Unable to convert hex input:', e)
        sys.exit(1)
        #e.args(e.args[0] + 'Unable to convert to hex input') 
        #raise

    haystack = ''
    for upper in ascii_uppercase:
        for lower in ascii_lowercase:
            for digit in digits:
                haystack += upper+lower+digit
                found_at = haystack.find(needle)
                if found_at > -1:
                    return found_at

    raise WasNotFoundException('Couldn`t find %s (%s) anywhere in the pattern.' %
          (search_pattern, needle))


def print_help():
    print('Usage: %s LENGTH|PATTERN' % sys.argv[0])
    print()
    print('Generate a pattern of length LENGTH or search for PATTERN and ')
    print('return its position in the pattern.')
    print()
    sys.exit(0)


if __name__ == '__main__':
    if len(sys.argv) < 2:
        print_help()

    if sys.argv[1].isdigit():
        pat = pattern_gen(int(sys.argv[1]))
        print(pat)
    else:
        found = pattern_search(sys.argv[1])
        print('Pattern %s first occurrence at position %d in pattern.' %
              (sys.argv[1], found))