How to convert an integer to the shortest url-safe string in Python?

I’d go the ‘encode integer as binary string, then base64 encode that’ method you suggest, and I’d do it using struct:

>>> import struct, base64
>>> base64.b64encode(struct.pack('l', 47))
'LwAAAA=='
>>> struct.unpack('l', base64.b64decode(_))
(47,)

Edit again:
To strip out the extra 0s on numbers that are too small to need full 32-bit precision, try this:

def pad(str, l=4):
    while len(str) < l:
        str = 'x00' + str
    return str

>>> base64.b64encode(struct.pack('!l', 47).replace('x00', ''))
'Lw=='
>>> struct.unpack('!l', pad(base64.b64decode('Lw==')))
(47,)

Base64 takes 4 bytes/characters to encode 3 bytes and can only encode multiples of 3 bytes (and adds padding otherwise).

So representing 4 bytes (your average int) in Base64 would take 8 bytes. Encoding the same 4 bytes in hex would also take 8 bytes. So you wouldn’t gain anything for a single int.

If you are looking for a way to shorten the integer representation using base64, I think you need to look elsewhere. When you encode something with base64 it doesn’t get shorter, in fact it gets longer.

E.g. 11234 encoded with base64 would yield MTEyMzQ=

When using base64 you have overlooked the fact that you are not converting just the digits (0-9) to a 64 character encoding. You are converting 3 bytes into 4 bytes so you are guaranteed your base64 encoded string would be 33.33% longer.

You don’t want base64 encoding, you want to represent a base 10 numeral in numeral base X.

If you want your base 10 numeral represented in the 26 letters available you could use: http://en.wikipedia.org/wiki/Hexavigesimal.
(You can extend that example for a much larger base by using all the legal url characters)

You should atleast be able to get base 38 (26 letters, 10 numbers, +, _)

The easy bit is converting the byte string to web-safe base64:

import base64
output = base64.urlsafe_b64encode(s)

The tricky bit is the first step – convert the integer to a byte string.

If your integers are small you’re better off hex encoding them – see saua

Otherwise (hacky recursive version):

def convertIntToByteString(i):
    if i == 0:
        return ""
    else:
        return convertIntToByteString(i >> 8) + chr(i & 255)

a little hacky, but it works:

def b64num(num_to_encode):
  h = hex(num_to_encode)[2:]     # hex(n) returns 0xhh, strip off the 0x
  h = len(h) & 1 and '0'+h or h  # if odd number of digits, prepend '0' which hex codec requires
  return h.decode('hex').encode('base64') 

you could replace the call to .encode(‘base64’) with something in the base64 module, such as urlsafe_b64encode()

To encode n:

data = ''
while n > 0:
    data = chr(n & 255) + data
    n = n >> 8
encoded = base64.urlsafe_b64encode(data).rstrip('=')

To decode s:

data = base64.urlsafe_b64decode(s + '===')
decoded = 0
while len(data) > 0:
    decoded = (decoded << 8) | ord(data[0])
    data = data[1:]

In the same spirit as other for some “optimal” encoding, you can use 73 characters according to RFC 1738 (actually 74 if you count “+” as usable):

alphabet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_`"!$'()*,-."
encoded = ''
while n > 0:
    n, r = divmod(n, len(alphabet))
    encoded = alphabet[r] + encoded

and the decoding:

decoded = 0
while len(s) > 0:
    decoded = decoded * len(alphabet) + alphabet.find(s[0])
    s = s[1:]

You probably do not want real base64 encoding for this – it will add padding etc, potentially even resulting in larger strings than hex would for small numbers. If there’s no need to interoperate with anything else, just use your own encoding. Eg. here’s a function that will encode to any base (note the digits are actually stored least-significant first to avoid extra reverse() calls:

def make_encoder(baseString):
    size = len(baseString)
    d = dict((ch, i) for (i, ch) in enumerate(baseString)) # Map from char -> value
    if len(d) != size:
        raise Exception("Duplicate characters in encoding string")

    def encode(x):
        if x==0: return baseString[0]  # Only needed if don't want '' for 0
        l=[]
        while x>0:
            l.append(baseString[x % size])
            x //= size
        return ''.join(l)

    def decode(s):
        return sum(d[ch] * size**i for (i,ch) in enumerate(s))

    return encode, decode

# Base 64 version:
encode,decode = make_encoder("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/")

assert decode(encode(435346456456)) == 435346456456

This has the advantage that you can use whatever base you want, just by adding appropriate
characters to the encoder’s base string.

Note that the gains for larger bases are not going to be that big however. base 64 will only reduce the size to 2/3rds of base 16 (6 bits/char instead of 4). Each doubling only adds one more bit per character. Unless you’ve a real need to compact things, just using hex will probably be the simplest and fastest option.

I maintain a little library named zbase62: http://pypi.python.org/pypi/zbase62

With it you can convert from a Python 2 str object to a base-62 encoded string and vice versa:

Python 2.7.1+ (r271:86832, Apr 11 2011, 18:13:53) 
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> d = os.urandom(32)
>>> d
'C$x8fxf9x92NVx97x13Hxc7Fx0cx0fx8d9}xf5.uxeeOrxc2Vx92fx1b=:xc3xbc'
>>> from zbase62 import zbase62
>>> encoded = zbase62.b2a(d)
>>> encoded
'Fv8kTvGhIrJvqQ2oTojUGlaVIxFE1b6BCLpH8JfYNRs'
>>> zbase62.a2b(encoded)
'C$x8fxf9x92NVx97x13Hxc7Fx0cx0fx8d9}xf5.uxeeOrxc2Vx92fx1b=:xc3xbc'

However, you still need to convert from integer to str. This comes built-in to Python 3:

Python 3.2 (r32:88445, Mar 25 2011, 19:56:22)
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> d = os.urandom(32)
>>> d
b'xe4x0bx94|xb6ox08xe9oRx1fxaaxa8xe8qS3x86x82tx15xf2"x1dL%?xdaxcc3xe3xba'
>>> int.from_bytes(d, 'big')
103147789615402524662804907510279354159900773934860106838120923694590497907642
>>> x= _ 
>>> x.to_bytes(32, 'big')
b'xe4x0bx94|xb6ox08xe9oRx1fxaaxa8xe8qS3x86x82tx15xf2"x1dL%?xdaxcc3xe3xba'

To convert from int to bytes and vice versa in Python 2, there is not a convenient, standard way as far as I know. I guess maybe I should copy some implementation, such as this one: https://github.com/warner/foolscap/blob/46e3a041167950fa93e48f65dcf106a576ed110e/foolscap/banana.py#L41 into zbase62 for your convenience.

I needed a signed integer, so I ended up going with:

import struct, base64

def b64encode_integer(i):
   return base64.urlsafe_b64encode(struct.pack('i', i)).rstrip('=n')

Example:

>>> b64encode_integer(1)
'AQAAAA'
>>> b64encode_integer(-1)
'_____w'
>>> b64encode_integer(256)
'AAEAAA'

All the answers given regarding Base64 are very reasonable solutions. But they’re technically incorrect. To convert an integer to the shortest URL safe string possible, what you want is base 66 (there are 66 URL safe characters).

That code looks something like this:

from io import StringIO
import urllib

BASE66_ALPHABET = u"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-_.~"
BASE = len(BASE66_ALPHABET)

def hexahexacontadecimal_encode_int(n):
    if n == 0:
        return BASE66_ALPHABET[0].encode('ascii')

    r = StringIO()
    while n:
        n, t = divmod(n, BASE)
        r.write(BASE66_ALPHABET[t])
    return r.getvalue().encode('ascii')[::-1]

Here’s a complete implementation of a scheme like this, ready to go as a pip installable package:

https://github.com/aljungberg/hhc

I’m working on making a pip package for this.

I recommend you use my bases.py https://github.com/kamijoutouma/bases.py which was inspired by bases.js

from bases import Bases
bases = Bases()

bases.toBase16(200)                // => 'c8'
bases.toBase(200, 16)              // => 'c8'
bases.toBase62(99999)              // => 'q0T'
bases.toBase(200, 62)              // => 'q0T'
bases.toAlphabet(300, 'aAbBcC')    // => 'Abba'

bases.fromBase16('c8')               // => 200
bases.fromBase('c8', 16)             // => 200
bases.fromBase62('q0T')              // => 99999
bases.fromBase('q0T', 62)            // => 99999
bases.fromAlphabet('Abba', 'aAbBcC') // => 300

refer to https://github.com/kamijoutouma/bases.py#known-basesalphabets
for what bases are usable

For your case

I recommend you use either base 32, 58 or 64

Base-64 warning: besides there being several different standards, padding isn’t currently added and line lengths aren’t tracked. Not recommended for use with APIs that expect formal base-64 strings!

Same goes for base 66 which is currently not supported by both bases.js and bases.py but it might in the future

Pure python, no dependancies, no encoding of byte strings etc. , just turning a base 10 int into base 64 int with the correct RFC 4648 characters:

def tetrasexagesimal(number):
    out=""
    while number>=0:
        if number == 0:
            out = 'A' + out
            break
        digit = number % 64
        out = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"[digit] + out
        number /= 64 # //= 64 for py3 (thank spanishgum!)
        if number == 0:
            break
    return out

tetrasexagesimal(1)

As it was mentioned here in comments you can encode a data using 73 characters that are not escaped in URL.
I found two places were this Base73 URL encoding is used:

• https://git.nolog.cz/NoLog.cz/f.bain/src/branch/master/static/script.js JS based URL shortener
• https://gist.github.com/LoneFry/3792021 in PHP

But in fact you may use more characters like /, [, ], :, ; and some others. Those characters are escaped only when you doing encodeURIComponent i.e. you need to pass data via get parameter.

So in fact you can use up to 82 characters. The full alphabet is !$&'()*+,-./0123456789:;=@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]_abcdefghijklmnopqrstuvwxyz~. I sorted all the symbols by their code so when Base82URL numbers are sorted as plain strings they are keep the same order.

I tested in Chrome and Firefox and they are works fine but may be confusing for regular users. But I used such ids for an internal API calls where nobody sees them.

Unsigned integer 32 bit may have a maximum value of 2^32=4294967296
And after encoding to the Base82 it will take 6 chars: $0~]mx.

I don’t have a code in Python but here is a JS code that generates a random id (int32 unsigned) and encodes it into the Base82URL:

        /**
         * Convert uint32 number to Base82 url safe
         * @param {int} number
         * @returns {string}
         */
        function toBase82Url(number) {
            // all chars that are not escaped in url
            let keys = "!$&'()*+,-./0123456789:;=@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]_abcdefghijklmnopqrstuvwxyz~"
            let radix = keys.length
            let encoded = []
            do {
                let index = number% radix
                encoded.unshift(keys.charAt(index))
                number = Math.trunc(number / radix)
            } while (number !== 0)
            return encoded .join("")
        }

        function generateToken() {
            let buf = new Uint32Array(1);
            window.crypto.getRandomValues(buf)
            var randomInt = buf[0]
            return toBase82Url(randomInt)
        }