Base 62 conversion in Python

Question

How would you convert an integer to base 62 (like hexadecimal, but with these digits: '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ').

I have been trying to find a good Python library for it, but they all seems to be occupied with converting strings. The Python base64 module only accepts strings and turns a single digit into four characters. I was looking for something akin to what URL shorteners use.

Answer 1

You probably want base64, not base62. There's an URL-compatible version of it floating around, so the extra two filler characters shouldn't be a problem.

The process is fairly simple; consider that base64 represents 6 bits and a regular byte represents 8. Assign a value from 000000 to 111111 to each of the 64 characters chosen, and put the 4 values together to match a set of 3 base256 bytes. Repeat for each set of 3 bytes, padding at the end with your choice of padding character (0 is generally useful).

Answer 2

Sorry, I can't help you with a library here. I would prefer using base64 and just adding to extra characters to your choice -- if possible!

Then you can use the base64 module.

If this is really, really not possible:

You can do it yourself this way (this is pseudo-code):

base62vals = []
myBase = 62
while num > 0:
   reminder = num % myBase
   num = num / myBase
   base62vals.insert(0, reminder)

Answer 3

There is no standard module for this, but I have written my own functions to achieve that.

ALPHABET = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

def base62_encode(num, alphabet=ALPHABET):
    """Encode a number in Base X

    `num`: The number to encode
    `alphabet`: The alphabet to use for encoding
    """
    if (num == 0):
        return alphabet[0]
    arr = []
    base = len(alphabet)
    while num:
        rem = num % base
        num = num // base
        arr.append(alphabet[rem])
    arr.reverse()
    return ''.join(arr)

def base62_decode(string, alphabet=ALPHABET):
    """Decode a Base X encoded string into the number

    Arguments:
    - `string`: The encoded string
    - `alphabet`: The alphabet to use for encoding
    """
    base = len(alphabet)
    strlen = len(string)
    num = 0

    idx = 0
    for char in string:
        power = (strlen - (idx + 1))
        num += alphabet.index(char) * (base ** power)
        idx += 1

    return num

Notice the fact that you can give it any Alphabet to use for encoding and decoding.

Hope this helps.

PS - For URL shorteners, I have found that it's better to leave out a few confusing characters like 0Ol1oI etc. Thus I use this alphabet for my URL shortening needs - "23456789abcdefghijkmnpqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ"

Have fun.

Answer 4

you can download zbase62 module from pypi

eg

>>> import zbase62
>>> zbase62.b2a("abcd")
'1mZPsa'

Answer 5

Personally I like the solution from Baishampayan, mostly because of stripping the confusing characters.

For completeness, and solution with better performance, this post shows a way to use the Python base64 module.

Answer 6

I have a Python library for doing exactly that here: http://www.djangosnippets.org/snippets/1431/

Answer 7

The following decoder-maker works with any reasonable base, has a much tidier loop, and gives an explicit error message when it meets an invalid character.

def base_n_decoder(alphabet):
    """Return a decoder for a base-n encoded string
    Argument:
    - `alphabet`: The alphabet used for encoding
    """
    base = len(alphabet)
    char_value = dict(((c, v) for v, c in enumerate(alphabet)))
    def f(string):
        num = 0
        try:
            for char in string:
                num = num * base + char_value[char]
        except KeyError:
            raise ValueError('Unexpected character %r' % char)
        return num
    return f

if __name__ == "__main__":
    func = base_n_decoder('0123456789abcdef')
    for test in ('0', 'f', '2020', 'ffff', 'abqdef'):
        print test
        print func(test)

Answer 8

I once wrote a script to do this aswell, I think it's quite elegant :)

import string
BASE_LIST = string.digits + string.letters + '_@'
BASE_DICT = dict((c, i) for i, c in enumerate(BASE_LIST))

def base_decode(string, reverse_base=BASE_DICT):
    length = len(reverse_base)
    ret = 0
    for i, c in enumerate(string[::-1]):
        ret += (length ** i) * reverse_base[c]

    return ret

def base_encode(integer, base=BASE_LIST):
    length = len(base)
    ret = ''
    while integer != 0:
        ret = base[integer % length] + ret
        integer /= length

    return ret

Example usage:

for i in range(100):                                    
    print i, base_decode(base_encode(i)), base_encode(i)