base64 encoding for integers. To make long numbers (i.e. hashes) more readable. Not as cool as RFC 1751, but less code.

base64.py

ALPHABET = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_'
TABLE = {c: i for (i, c) in enumerate(ALPHABET)}

def decode(s, b=64):
    """Interpret the input string as a base b number"""
    i, multiplier = 0, 1
    for c in reversed(s):
        i += multiplier * TABLE[c]
        multiplier *= b
    return i
        
        
def encode(h, b=64):
    """Given an hex string, convert to a base b number."""
    assert b <= len(ALPHABET)
    i = int(h, 16)  # hex -> decimal
    s = ''
    while 0 < i:
        i, d = divmod(i, b)
        s += ALPHABET[d]
    return s[::-1]

Why not use the standard order and just change the +/ to -_?

ALPHABET='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'  #or
ALPHABET='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_'

I recommend (since b is a parameter) that the ALPHABET and TABLE actually be indexed by b like:

ALPHABET={
    56: '23456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnpqrstuvwxyz',
    58: '123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz',
    64: 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/',
    '64u': 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_',
}
TABLE={b:{c: i for (i,c) in enumerate(ALPHABET[b])} for b in ALPHABET}