Python base 36 encoding

I wish I had read this before. Here is the answer:

def base36encode(number):
    if not isinstance(number, (int, long)):
        raise TypeError('number must be an integer')
    is_negative = number < 0
    number = abs(number)

    alphabet, base36 = ['0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ', '']

    while number:
        number, i = divmod(number, 36)
        base36 = alphabet[i] + base36
    if is_negative:
        base36 = '-' + base36

    return base36 or alphabet[0]


def base36decode(number):
    return int(number, 36)

print(base36encode(1412823931503067241))
print(base36decode('AQF8AA0006EH'))
assert(base36decode(base36encode(-9223372036721928027)) == -9223372036721928027)

terrible answer, but was just playing around with this an thought i’d share.

import string, math

int2base = lambda a, b: ''.join(
    [(string.digits +
      string.ascii_lowercase +
      string.ascii_uppercase)[(a // b ** i) % b]
     for i in range(int(math.log(a, b)), -1, -1)]
)

num = 1412823931503067241
test = int2base(num, 36)
test2 = int(test, 36)
print test2 == num

This works if you only care about positive integers.

def int_to_base36(num):
    """Converts a positive integer into a base36 string."""
    assert num >= 0
    digits = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'

    res = ''
    while not res or num > 0:
        num, i = divmod(num, 36)
        res = digits[i] + res
    return res

To convert back to int, just use int(num, 36). For a conversion of arbitrary bases see https://gist.github.com/mbarkhau/1b918cb3b4a2bdaf841c

from numpy import base_repr

num = base_repr(num, 36)
num = int(num, 36)

Here is information about numpy.base_repr.

You could use https://github.com/tonyseek/python-base36.

$ pip install base36

and then

>>> import base36
>>> assert base36.dumps(19930503) == 'bv6h3'
>>> assert base36.loads('bv6h3') == 19930503

You can use numpy’s base_repr(...) for this.

import numpy as np

num = 2017

num = np.base_repr(num, 36)
print(num)  # 1K1

num = int(num, 36)
print(num)  # 2017

Here is some information about numpy, int(x, base=10), and np.base_repr(number, base=2, padding=0).

(This answer was originally submitted as an edit to @christopher-beland’s answer, but was rejected in favor of its own answer.)

I benchmarked the example encoders provided in answers to this question. On my Ubuntu 18.10 laptop, Python 3.7, Jupyter, the %%timeit magic command, and the integer 4242424242424242 as the input, I got these results:

• Wikipedia’s sample code: 4.87 µs ± 300 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
• @mistero’s base36encode(): 3.62 µs ± 44.2 ns per loop
• @user1036542’s int2base: 10 µs ± 400 ns per loop (after fixing py37 compatibility)
• @mbarkhau’s int_to_base36(): 3.83 µs ± 28.8 ns per loop

All timings were mean ± std. dev. of 7 runs, 100000 loops each.

If you are feeling functional

def b36_encode(i):
    if i < 0: return "-" + b36_encode(-i)
    if i < 36: return "0123456789abcdefghijklmnopqrstuvwxyz"[i]
    return b36_encode(i // 36) + b36_encode(i % 36)    

test

n = -919283471029384701938478
s = "-45p3wubacgd6s0fi"
assert int(s, base=36) == n
assert b36_encode(n) == s