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

Question

I want the shortest possible way of representing an integer in a URL. For example, 11234 can be shortened to '2be2' using hexadecimal. Since base64 uses is a 64 character encoding, it should be possible to represent an integer in base64 using even less characters than hexadecimal. The problem is I can't figure out the cleanest way to convert an integer to base64 (and back again) using Python.

The base64 module has methods for dealing with bytestrings - so maybe one solution would be to convert an integer to its binary representation as a Python string... but I'm not sure how to do that either.

Answer 1

This answer is similar in spirit to Douglas Leeder's, with the following changes:

• It doesn't use actual Base64, so there's no padding characters

• Instead of converting the number first to a byte-string (base 256), it converts it directly to base 64, which has the advantage of letting you represent negative numbers using a sign character.

  import string
  ALPHABET = string.ascii_uppercase + string.ascii_lowercase + 
             string.digits + '-_'
  ALPHABET_REVERSE = dict((c, i) for (i, c) in enumerate(ALPHABET))
  BASE = len(ALPHABET)
  SIGN_CHARACTER = '$'
  
  def num_encode(n):
      if n < 0:
          return SIGN_CHARACTER + num_encode(-n)
      s = []
      while True:
          n, r = divmod(n, BASE)
          s.append(ALPHABET[r])
          if n == 0: break
      return ''.join(reversed(s))
  
  def num_decode(s):
      if s[0] == SIGN_CHARACTER:
          return -num_decode(s[1:])
      n = 0
      for c in s:
          n = n * BASE + ALPHABET_REVERSE[c]
      return n
  

---

    >>> num_encode(0)
    'A'
    >>> num_encode(64)
    'BA'
    >>> num_encode(-(64**5-1))
    '$_____'

---

A few side notes:

• You could (marginally) increase the human-readibility of the base-64 numbers by putting string.digits first in the alphabet (and making the sign character '-'); I chose the order that I did based on Python's urlsafe_b64encode.
• If you're encoding a lot of negative numbers, you could increase the efficiency by using a sign bit or one's/two's complement instead of a sign character.
• You should be able to easily adapt this code to different bases by changing the alphabet, either to restrict it to only alphanumeric characters or to add additional "URL-safe" characters.
• I would recommend against using a representation other than base 10 in URIs in most cases—it adds complexity and makes debugging harder without significant savings compared to the overhead of HTTP—unless you're going for something TinyURL-esque.