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Date: Thu, 11 Apr 2013 15:53:30 +1000
Subject: Re: performance of script to write very long lines of random chars
From: Chris Angelico <rosuav@gmail.com>
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Xref: csiph.com comp.lang.python:43320

On Thu, Apr 11, 2013 at 3:33 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> I was originally going to write that using the base64 module would
> introduce bias into the random strings, but after a little investigation,
> I don't think it does.

Assuming that os.urandom() returns bytes with perfectly fair
distribution (exactly equal chance of any value 00-FF - it probably
does, or close to it), and assuming that you work with exact multiples
of 3 bytes and 4 output characters, base64 will give you perfectly
fair distribution of result characters. You take three bytes (24 bits)
and turn them into four characters (6 bits per character, = 24 bits).
You might see some bias if you use less than a full set of four output
characters, though; I haven't dug into the details to check that.

ChrisA