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To: python-list@python.org
From: Ned Deily <nad@acm.org>
Subject: Re: python 3.44 float addition bug?
Date: Sat, 21 Jun 2014 12:24:24 -0700
References: <XnsA35313E634BA0fraserlonggmailcom34@216.196.109.145> <CAPTjJmrkPd5K__h9Qg12Q+AraFZVaN6eGUdTmEDGe2ccAqEmRw@mail.gmail.com>
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In article 
<CAPTjJmrkPd5K__h9Qg12Q+AraFZVaN6eGUdTmEDGe2ccAqEmRw@mail.gmail.com>,
 Chris Angelico <rosuav@gmail.com> wrote:
> Also, when you're looking at how things print out, consider looking at
> two things: the str() and the repr(). Sometimes just "print(p)"
> doesn't give you all the info, so you might instead want to write your
> loop thus:
> 
> z = 0.01
> p = 0.0
> for i in range(19):
>     p += z
>     print(str(p) + " -- " + repr(p)) 
> Sometimes you can get extra clues that way, although in this instance
> I think you won't.

Actually, I think this is one case where you would get extra clues (or 
extra headscratching) if you run the code with various releases of 
Python.

$ python2.6 b.py
0.01 -- 0.01
0.02 -- 0.02
0.03 -- 0.029999999999999999
0.04 -- 0.040000000000000001
0.05 -- 0.050000000000000003
0.06 -- 0.060000000000000005
0.07 -- 0.070000000000000007
0.08 -- 0.080000000000000002
0.09 -- 0.089999999999999997
0.1 -- 0.099999999999999992
0.11 -- 0.10999999999999999
0.12 -- 0.11999999999999998
0.13 -- 0.12999999999999998
0.14 -- 0.13999999999999999
0.15 -- 0.14999999999999999
0.16 -- 0.16
0.17 -- 0.17000000000000001
0.18 -- 0.18000000000000002
0.19 -- 0.19000000000000003

$ python2.7 b.py
0.01 -- 0.01
0.02 -- 0.02
0.03 -- 0.03
0.04 -- 0.04
0.05 -- 0.05
0.06 -- 0.060000000000000005
0.07 -- 0.07
0.08 -- 0.08
0.09 -- 0.09
0.1 -- 0.09999999999999999
0.11 -- 0.10999999999999999
0.12 -- 0.11999999999999998
0.13 -- 0.12999999999999998
0.14 -- 0.13999999999999999
0.15 -- 0.15
0.16 -- 0.16
0.17 -- 0.17
0.18 -- 0.18000000000000002
0.19 -- 0.19000000000000003

$ python3.4 b.py
0.01 -- 0.01
0.02 -- 0.02
0.03 -- 0.03
0.04 -- 0.04
0.05 -- 0.05
0.060000000000000005 -- 0.060000000000000005
0.07 -- 0.07
0.08 -- 0.08
0.09 -- 0.09
0.09999999999999999 -- 0.09999999999999999
0.10999999999999999 -- 0.10999999999999999
0.11999999999999998 -- 0.11999999999999998
0.12999999999999998 -- 0.12999999999999998
0.13999999999999999 -- 0.13999999999999999
0.15 -- 0.15
0.16 -- 0.16
0.17 -- 0.17
0.18000000000000002 -- 0.18000000000000002
0.19000000000000003 -- 0.19000000000000003

What's going on here is that in Python 2.7 the repr() of floats was 
changed to use the minimum number of digits to accurately roundtrip the 
number under correct rounding.  For compatibility reasons, the str() 
representation was not changed for 2.7.  But in Python 3.2, str() was 
changed to be identical to repr() for floats.  It's important to keep in 
mind that the actual binary values stored in float objects are the same 
across all of these releases; only the representation of them as decimal 
characters varies.

https://docs.python.org/2.7/whatsnew/2.7.html#other-language-changes

http://bugs.python.org/issue9337

-- 
 Ned Deily,
 nad@acm.org