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    <p>This answer is similar in spirit to Douglas Leeder's, with the following changes:</p> <ul> <li>It doesn't use actual Base64, so there's no padding characters</li> <li><p>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.</p> <pre><code>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 = '$' def num_encode(n): if n &lt; 0: return SIGN_CHARACTER + num_encode(-n) s = [] while True: n, r = divmod(n, BASE) s.append(ALPHABET[r]) if n == 0: break return ''.join(reversed(s)) def num_decode(s): if s[0] == SIGN_CHARACTER: return -num_decode(s[1:]) n = 0 for c in s: n = n * BASE + ALPHABET_REVERSE[c] return n </code></pre></li> </ul> <hr> <pre><code> &gt;&gt;&gt; num_encode(0) 'A' &gt;&gt;&gt; num_encode(64) 'BA' &gt;&gt;&gt; num_encode(-(64**5-1)) '$_____' </code></pre> <hr> <p>A few side notes:</p> <ul> <li>You could (<em>marginally</em>) 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.</li> <li>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.</li> <li>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.</li> <li>I would recommend <em>against</em> 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.</li> </ul>
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    1. POHow to convert an integer to the shortest url-safe string in Python?
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    1. POHow to convert an integer to the shortest url-safe string in Python?
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    1. COVoted up to have thought about negative numbers. But isn’t one byte for the sign a bit expensive?
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    2. COYes, it is, which I addressed somewhat in my second note; but if that's not a concern, the implementation using a sign character was the simplest ;)
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    3. COThe initial place I want to use this is "recover your account" style URLs which include a user ID, a timestamp and an sha1 hash - and should ideally be less than 80 characters to ensure they can be safely e-mailed without text wrapping screwing them up.
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      1. USSimon Willison
 

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