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Bug in floating point multiplication

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  Bug in floating point multiplication Steven D'Aprano <steve@pearwood.info> - 2015-07-03 00:52 +1000
    Re: Bug in floating point multiplication Michael Poeltl <michael.poeltl@univie.ac.at> - 2015-07-02 17:10 +0200
    Re: Bug in floating point multiplication Chris Angelico <rosuav@gmail.com> - 2015-07-03 01:21 +1000
    Re: Bug in floating point multiplication Robin Becker <robin@reportlab.com> - 2015-07-02 16:24 +0100
    Re: Bug in floating point multiplication Paul Rubin <no.email@nospam.invalid> - 2015-07-02 08:26 -0700
      Re: Bug in floating point multiplication Chris Angelico <rosuav@gmail.com> - 2015-07-03 01:34 +1000
        Re: Bug in floating point multiplication Steven D'Aprano <steve@pearwood.info> - 2015-07-03 01:41 +1000
      Re: Bug in floating point multiplication Ian Kelly <ian.g.kelly@gmail.com> - 2015-07-02 09:38 -0600
    Re: Bug in floating point multiplication Vincent Vande Vyvre <vincent.vande.vyvre@telenet.be> - 2015-07-02 17:15 +0200
    Re: Bug in floating point multiplication Ian Kelly <ian.g.kelly@gmail.com> - 2015-07-02 09:28 -0600
    Re: Bug in floating point multiplication Robin Becker <robin@reportlab.com> - 2015-07-02 16:29 +0100
    Re: Bug in floating point multiplication Ian Kelly <ian.g.kelly@gmail.com> - 2015-07-02 09:32 -0600
    Re: Bug in floating point multiplication Steven D'Aprano <steve@pearwood.info> - 2015-07-03 01:42 +1000
    Re: Bug in floating point multiplication duncan smith <buzzard@invalid.invalid> - 2015-07-02 17:08 +0100
    Re: Bug in floating point multiplication MRAB <python@mrabarnett.plus.com> - 2015-07-02 17:20 +0100
    Re: Bug in floating point multiplication Tim Chase <python.list@tim.thechases.com> - 2015-07-02 11:59 -0500
    Re: Bug in floating point multiplication Irmen de Jong <irmen.NOSPAM@xs4all.nl> - 2015-07-02 19:49 +0200
      Re: Bug in floating point multiplication Ned Deily <nad@acm.org> - 2015-07-02 22:07 -0700
        Re: Bug in floating point multiplication Irmen de Jong <irmen.NOSPAM@xs4all.nl> - 2015-07-03 18:27 +0200
    Re: Bug in floating point multiplication Laurent Pointal <laurent.pointal@free.fr> - 2015-07-02 23:12 +0200
    Re: Bug in floating point multiplication Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2015-07-03 16:13 +0100
    Re: Bug in floating point multiplication Laura Creighton <lac@openend.se> - 2015-07-05 00:54 +0200
    Re: Bug in floating point multiplication Peter Otten <__peter__@web.de> - 2015-07-05 09:38 +0200

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#93425 — Bug in floating point multiplication

Despite the title, this is not one of the usual "Why can't Python do
maths?" "bug" reports.

Can anyone reproduce this behaviour? If so, please reply with the version of
Python and your operating system. Printing sys.version will probably do.


x = 1 - 1/2**53
assert x == 0.9999999999999999
for i in range(1, 1000000):
    if int(i*x) == i:
        print(i); break


Using Jython and IronPython, the loop runs to completion. That is the
correct behaviour, or so I am lead to believe. Using Python 2.6, 2.7 and
3.3 on Centos and Debian, it prints 2049 and breaks. That should not
happen. If you can reproduce that (for any value of i, not necessarily
2049), please reply.

See also http://bugs.python.org/issue24546 for more details.



-- 
Steven

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#93428

hi Steven,

I'm running python-3.4.2 on a linuxmint16 box and CANNOT reproduce
it is just that
int(i*x) == i
is never True!

hope that helps
regards
Michael

* Steven D'Aprano <steve@pearwood.info> [2015-07-02 16:56]:
> Despite the title, this is not one of the usual "Why can't Python do
> maths?" "bug" reports.
> 
> Can anyone reproduce this behaviour? If so, please reply with the version of
> Python and your operating system. Printing sys.version will probably do.
> 
> 
> x = 1 - 1/2**53
> assert x == 0.9999999999999999
> for i in range(1, 1000000):
>     if int(i*x) == i:
>         print(i); break
> 
> 
> Using Jython and IronPython, the loop runs to completion. That is the
> correct behaviour, or so I am lead to believe. Using Python 2.6, 2.7 and
> 3.3 on Centos and Debian, it prints 2049 and breaks. That should not
> happen. If you can reproduce that (for any value of i, not necessarily
> 2049), please reply.
> 
> See also http://bugs.python.org/issue24546 for more details.
> 
> 
> 
> -- 
> Steven
> 
> -- 
> https://mail.python.org/mailman/listinfo/python-list

-- 
  Michael Poeltl <michael.poeltl@univie.ac.at>
  Computational Materials Physics at University
  Wien, Sensengasse 8/12, A-1090 Wien, AUSTRIA
  http://cmp.univie.ac.at/
  http://homepage.univie.ac.at/michael.poeltl/
  using elinks-0.12, mutt-1.5.21, and vim-7.3,
  with python-3.4.2, on linux mint 16 (petra)   :-)
  fon: +43-1-4277-51409

  "Lehrend lernen wir!"

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#93429

On Fri, Jul 3, 2015 at 12:52 AM, Steven D'Aprano <steve@pearwood.info> wrote:
> Can anyone reproduce this behaviour? If so, please reply with the version of
> Python and your operating system. Printing sys.version will probably do.
>
>
> x = 1 - 1/2**53
> assert x == 0.9999999999999999
> for i in range(1, 1000000):
>     if int(i*x) == i:
>         print(i); break
>

I suspect for Py2 you need to say 1/2.0, or insist on a future
directive. Otherwise the assertion fails, x is simply 1, and the loop
instantly halts, none of which is buggy behaviour.

Testing on a 64-bit Debian Jessie, these all run to completion:
Python 3.6.0a0 (default:5c901b39c6b7, Jun 28 2015, 09:13:39)
Python 3.5.0b1+ (default:7255af1a1c50+, May 26 2015, 00:39:06)
Python 3.4.2 (default, Oct  8 2014, 10:45:20)
Python 2.7.9 (default, Mar  1 2015, 12:57:24)

Testing on a 32-bit Debian Jessie, these all print 2049 and stop:
Python 3.4.2 (default, Oct  8 2014, 13:14:40)
Python 3.3.3 (default, Dec 31 2013, 19:11:08)
Python 2.7.9 (default, Mar  1 2015, 18:22:53)
Python 2.6.6 (r266:84292, Dec 27 2010, 00:02:40)

(2.6.6 isn't actually supported on Jessie, it just happens to be laying around.)

On 32-bit AntiX, these both print 2049 and stop:
Python 3.4.3+ (default, Jun  2 2015, 14:09:35)
Python 2.7.10 (default, Jun  1 2015, 16:21:46)

Looks like it's an issue with 32-bit builds only. And not on Windows -
I spun up a Win 7 VM and tested it to completion:

Python 3.4.3 (v3.4.3:9b73f1c3e601, Feb 24 2015, 22:43:06) [MSC v.1600
32 bit (Intel)] on win32
Python 2.7.10 (default, May 23 2015, 09:40:32) [MSC v.1500 32 bit
(Intel)] on win32

Over-eager optimization somewhere?

ChrisA

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#93430

On 02/07/2015 15:52, Steven D'Aprano wrote:
> Despite the title, this is not one of the usual "Why can't Python do
> maths?" "bug" reports.
>
> Can anyone reproduce this behaviour? If so, please reply with the version of
> Python and your operating system. Printing sys.version will probably do.
>
>
> x = 1 - 1/2**53
> assert x == 0.9999999999999999
...

as stated the above fails since it produces x==1L for python 2.7 (Python 2.7.8 
(default, Jun 30 2014, 16:08:48) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.)

If I convert old style

x = 1.0 - 1.0/2**53 then the assertion succeeds and the loop does not print.
-- 
Robin Becker

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#93431

Steven D'Aprano <steve@pearwood.info> writes:
> x = 1 - 1/2**53
> assert x == 0.9999999999999999

In Python 2.x I don't see how that assert can possibly succeed, since
x is the integer 1.  But I tested it anyway on 2.7.5 under Fedora 19
and it threw an assertion error.

I changed it to say 1 - 1/2.0**53 and then the loop runs to completion.

sys.version is:
2.7.5 (default, Nov  3 2014, 14:33:39) \n[GCC 4.8.3 20140911 (Red Hat 4.8.3-7)]

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#93436

On Fri, Jul 3, 2015 at 1:26 AM, Paul Rubin <no.email@nospam.invalid> wrote:
> Steven D'Aprano <steve@pearwood.info> writes:
>> x = 1 - 1/2**53
>> assert x == 0.9999999999999999
>
> In Python 2.x I don't see how that assert can possibly succeed, since
> x is the integer 1.  But I tested it anyway on 2.7.5 under Fedora 19
> and it threw an assertion error.

>From previous discussions I happen to know that Steven normally runs
everything with "from __future__ import division" active (and possibly
others? not sure), so just assume he means to work with floats here.
Steven, I think this is one of the downsides of using future
directives by default - you forget that your code can't always be
copied and pasted to other people :)

ChrisA

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#93438

On Fri, 3 Jul 2015 01:34 am, Chris Angelico wrote:

> From previous discussions I happen to know that Steven normally runs
> everything with "from __future__ import division" active (and possibly
> others? not sure), so just assume he means to work with floats here.
> Steven, I think this is one of the downsides of using future
> directives by default - you forget that your code can't always be
> copied and pasted to other people :)

/me hangs his head in shame

Yes, you've diagnosed the problem correctly. Sorry for the confusion.



-- 
Steven

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#93437

On Thu, Jul 2, 2015 at 9:26 AM, Paul Rubin <no.email@nospam.invalid> wrote:
> Steven D'Aprano <steve@pearwood.info> writes:
>> x = 1 - 1/2**53
>> assert x == 0.9999999999999999
>
> In Python 2.x I don't see how that assert can possibly succeed, since
> x is the integer 1.  But I tested it anyway on 2.7.5 under Fedora 19
> and it threw an assertion error.

Oh, duh. That was the problem on my 2.7.6 test as well. I added from
__future__ import division, and now it runs to completion.

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#93432

Le 02/07/2015 16:52, Steven D'Aprano a écrit :
> Despite the title, this is not one of the usual "Why can't Python do
> maths?" "bug" reports.
>
> Can anyone reproduce this behaviour? If so, please reply with the version of
> Python and your operating system. Printing sys.version will probably do.
>
>
> x = 1 - 1/2**53
> assert x == 0.9999999999999999
> for i in range(1, 1000000):
>      if int(i*x) == i:
>          print(i); break
>
>
> Using Jython and IronPython, the loop runs to completion. That is the
> correct behaviour, or so I am lead to believe. Using Python 2.6, 2.7 and
> 3.3 on Centos and Debian, it prints 2049 and breaks. That should not
> happen. If you can reproduce that (for any value of i, not necessarily
> 2049), please reply.
>
> See also http://bugs.python.org/issue24546 for more details.
>
>
>

Hi,

vincent@tiemoko:~$ python3
Python 3.2.3 (default, Jun 18 2015, 21:46:42)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> x = 1 - 1/2**53
 >>> assert x == 0.9999999999999999
 >>> for i in range(1, 1000000):
...     if int(i*x) == i:
...             print(i)
...             break
...
2049
 >>>

-------------------------------------------------------------------------------
vincent@djoliba:~$ python3
Python 3.4.0 (default, Jun 19 2015, 14:20:21)
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
 >>> x = 1 - 1/2**53
 >>> assert x == 0.9999999999999999
 >>> for i in range(1, 1000000):
...     if int(i*x) == i:
...         print(i)
...         break
...
 >>>

Both on Ubuntu.

Vincent

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#93433

On my Chromebook, using Python 2.7.6 from the Ubuntu Trusty
distribution, I get AssertionError, and x == 1.

In Python 3.4.0 on the same system, the code runs to completion. Both
Pythons appear to be 64-bit builds.

On my Mint 17.1 desktop (which should be using the same packages), I
get the same results.

On Thu, Jul 2, 2015 at 8:52 AM, Steven D'Aprano <steve@pearwood.info> wrote:
> Despite the title, this is not one of the usual "Why can't Python do
> maths?" "bug" reports.
>
> Can anyone reproduce this behaviour? If so, please reply with the version of
> Python and your operating system. Printing sys.version will probably do.
>
>
> x = 1 - 1/2**53
> assert x == 0.9999999999999999
> for i in range(1, 1000000):
>     if int(i*x) == i:
>         print(i); break
>
>
> Using Jython and IronPython, the loop runs to completion. That is the
> correct behaviour, or so I am lead to believe. Using Python 2.6, 2.7 and
> 3.3 on Centos and Debian, it prints 2049 and breaks. That should not
> happen. If you can reproduce that (for any value of i, not necessarily
> 2049), please reply.
>
> See also http://bugs.python.org/issue24546 for more details.
>
>
>
> --
> Steven
>
> --
> https://mail.python.org/mailman/listinfo/python-list

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#93434

$ uname -a
Linux everest 4.0.6-1-ARCH #1 SMP PREEMPT Tue Jun 23 14:40:31 CEST 2015 i686 
GNU/Linux
robin@everest:~
$ python2
Python 2.7.10 (default, May 26 2015, 04:28:58)
[GCC 5.1.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> x = 1.0 - 1.0/2**53
 >>> assert x == 0.9999999999999999
 >>> for i in range(1, 1000000):
...     if int(i*x) == i:
...         print(i); break
...
2049
 >>>
robin@everest:~
$ python
Python 3.4.3 (default, Mar 26 2015, 07:36:01)
[GCC 4.9.2 20150304 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
 >>>
KeyboardInterrupt
 >>> x = 1 - 1/2**53
 >>> assert x == 0.9999999999999999
 >>>  for i in range(1, 1000000):
   File "<stdin>", line 1
     for i in range(1, 1000000):
     ^
IndentationError: unexpected indent
 >>> for i in range(1, 1000000):
...     if int(i*x) == i:
...          print(i); break
...
2049
 >>>
robin@everest:~/devel/filps




-- 
Robin Becker

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#93435

On Thu, Jul 2, 2015 at 9:28 AM, Ian Kelly <ian.g.kelly@gmail.com> wrote:
> On my Chromebook, using Python 2.7.6 from the Ubuntu Trusty
> distribution, I get AssertionError, and x == 1.
>
> In Python 3.4.0 on the same system, the code runs to completion. Both
> Pythons appear to be 64-bit builds.
>
> On my Mint 17.1 desktop (which should be using the same packages), I
> get the same results.

Apologies for the accidental top-posting. Also, the full sys.version
string for the 2.7.6 Python is:

2.7.6 (default, Jun 22 2015, 17:58:13)
[GCC 4.8.2]

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#93439

On Fri, 3 Jul 2015 12:52 am, Steven D'Aprano wrote:


> x = 1 - 1/2**53

Ooops, sorry I forgot that I had already run "from __future__ import
division". Otherwise that line needs to be:

x = 1 - 1.0/2**53



-- 
Steven

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#93441

On 02/07/15 15:52, Steven D'Aprano wrote:
> Despite the title, this is not one of the usual "Why can't Python do
> maths?" "bug" reports.
> 
> Can anyone reproduce this behaviour? If so, please reply with the version of
> Python and your operating system. Printing sys.version will probably do.
> 
> 
> x = 1 - 1/2**53
> assert x == 0.9999999999999999
> for i in range(1, 1000000):
>     if int(i*x) == i:
>         print(i); break
> 
> 
> Using Jython and IronPython, the loop runs to completion. That is the
> correct behaviour, or so I am lead to believe. Using Python 2.6, 2.7 and
> 3.3 on Centos and Debian, it prints 2049 and breaks. That should not
> happen. If you can reproduce that (for any value of i, not necessarily
> 2049), please reply.
> 
> See also http://bugs.python.org/issue24546 for more details.
> 
> 
> 

Python 2.7.6 (default, Jun 22 2015, 17:58:13)
[GCC 4.8.2] on linux2
Type "copyright", "credits" or "license()" for more information.
>>> from __future__ import division
>>> x = 1 - 1/2**53
>>> assert x == 0.9999999999999999
>>> for i in range(1, 1000000):
	if int(i*x) == i:
		print(i); break

		
>>>


Python 3.4.0 (default, Jun 19 2015, 14:20:21)
[GCC 4.8.2] on linux
Type "copyright", "credits" or "license()" for more information.
>>> x = 1 - 1/2**53
>>> assert x == 0.9999999999999999
>>> for i in range(1, 1000000):
	if int(i*x) == i:
		print(i); break

		
>>>


Ubuntu 14.04

Duncan

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#93443

On 2015-07-02 15:52, Steven D'Aprano wrote:
> Despite the title, this is not one of the usual "Why can't Python do
> maths?" "bug" reports.
>
> Can anyone reproduce this behaviour? If so, please reply with the version of
> Python and your operating system. Printing sys.version will probably do.
>
>
> x = 1 - 1/2**53
> assert x == 0.9999999999999999
> for i in range(1, 1000000):
>      if int(i*x) == i:
>          print(i); break
>
>
> Using Jython and IronPython, the loop runs to completion. That is the
> correct behaviour, or so I am lead to believe. Using Python 2.6, 2.7 and
> 3.3 on Centos and Debian, it prints 2049 and breaks. That should not
> happen. If you can reproduce that (for any value of i, not necessarily
> 2049), please reply.
>
> See also http://bugs.python.org/issue24546 for more details.
>
These are all on Windows 8.1 with CPython from www.python.org.

With "x = 1 - 1.0/2**53" (needed for Python 2.x) they all passed.

2.5.4 (r254:67916, Dec 23 2008, 15:19:34) [MSC v.1400 64 bit (AMD64)]
2.5.2 (r252:60911, Feb 21 2008, 13:11:45) [MSC v.1310 32 bit (Intel)]
2.6.6 (r266:84297, Aug 24 2010, 18:13:38) [MSC v.1500 64 bit (AMD64)]
2.6.5 (r265:79096, Mar 19 2010, 21:48:26) [MSC v.1500 32 bit (Intel)]
2.7.7 (default, Jun  1 2014, 14:21:57) [MSC v.1500 64 bit (AMD64)]
2.7.7 (default, Jun  1 2014, 14:17:13) [MSC v.1500 32 bit (Intel)]
3.1.4 (default, Jun 12 2011, 14:16:16) [MSC v.1500 64 bit (AMD64)]
3.1.4 (default, Jun 12 2011, 15:05:44) [MSC v.1500 32 bit (Intel)]
3.2.5 (default, May 15 2013, 23:07:10) [MSC v.1500 64 bit (AMD64)]
3.2.3 (default, Apr 11 2012, 07:15:24) [MSC v.1500 32 bit (Intel)]
3.3.5 (v3.3.5:62cf4e77f785, Mar  9 2014, 10:35:05) [MSC v.1600 64 bit 
(AMD64)]
3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:03:43) [MSC v.1600 32 bit 
(Intel)]
3.4.2 (v3.4.2:ab2c023a9432, Oct  6 2014, 22:16:31) [MSC v.1600 64 bit 
(AMD64)]
3.4.2 (v3.4.2:ab2c023a9432, Oct  6 2014, 22:15:05) [MSC v.1600 32 bit 
(Intel)]
3.5.0b2 (v3.5.0b2:7a088af5615b, May 31 2015, 06:22:19) [MSC v.1900 64 
bit (AMD64)]
3.5.0b2 (v3.5.0b2:7a088af5615b, May 31 2015, 06:08:44) [MSC v.1900 32 
bit (Intel)]

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#93446

On 2015-07-03 00:52, Steven D'Aprano wrote:
> x = 1 - 1/2**53
> assert x == 0.9999999999999999
> for i in range(1, 1000000):
>     if int(i*x) == i:
>         print(i); break

tkc@debian:~$ python
Python 2.7.9 (default, Mar  1 2015, 12:57:24) 
[GCC 4.9.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> x = 1 - 1/2**53
>>> assert x == 0.9999999999999999
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError
>>> for i in range(1, 1000000):
...     if int(i*x) == i:
...         print(i); break
... 
1
>>> x
1
>>> ^D
tkc@debian:~$ python3
Python 3.4.2 (default, Oct  8 2014, 10:45:20) 
[GCC 4.9.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> x = 1 - 1/2**53
>>> assert x == 0.9999999999999999
>>> for i in range(1, 1000000):
...     if int(i*x) == i:
...         print(i); break
... 
>>> ^D

tkc@debian:~$ uname -a
Linux bigbox 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt9-3~deb8u1
(2015-04-24) x86_64 GNU/Linux

Weird that on 2.7, the assert fails.

-tkc

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#93448

On 2-7-2015 16:52, Steven D'Aprano wrote:
> Despite the title, this is not one of the usual "Why can't Python do
> maths?" "bug" reports.
> 
> Can anyone reproduce this behaviour? If so, please reply with the version of
> Python and your operating system. Printing sys.version will probably do.
> 
> 
> x = 1 - 1/2**53
> assert x == 0.9999999999999999
> for i in range(1, 1000000):
>     if int(i*x) == i:
>         print(i); break

I can't reproduce it with any of the pythons I have here, they all run to completion:

Tested on Mac OSX 10.10.4, with a 64-bit core2duo processor. Below are all 64-bit python
implementations:
2.6.9 (apple supplied), 2.7.6 (apple supplied), 3.4.3 (homebrew), and pypy-2.6.0
(homebrew). I don't have any 32 bit Python implementations on the mac.

Tested on windows 7, with a 64-bit core2quad processor, but using 32-bit python
implementations from python.org: 2.7.9, 3.3.3, 3.4.2, and pypy-2.5.1.


Irmen de Jong

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#93460

In article <559579bb$0$2921$e4fe514c@news.xs4all.nl>,
 Irmen de Jong <irmen.NOSPAM@xs4all.nl> wrote:
> Tested on Mac OSX 10.10.4, with a 64-bit core2duo processor. Below are all 
> 64-bit python
> implementations:
> 2.6.9 (apple supplied), 2.7.6 (apple supplied), 3.4.3 (homebrew), and 
> pypy-2.6.0
> (homebrew). I don't have any 32 bit Python implementations on the mac.

Sure you do!

$ /usr/bin/python2.7 -c 'import sys;print(sys.maxsize)'
9223372036854775807
$ arch -i386 /usr/bin/python2.7 -c 'import sys;print(sys.maxsize)'
2147483647

-- 
 Ned Deily,
 nad@acm.org

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#93467

On 3-7-2015 7:07, Ned Deily wrote:
> In article <559579bb$0$2921$e4fe514c@news.xs4all.nl>,
>  Irmen de Jong <irmen.NOSPAM@xs4all.nl> wrote:
>> Tested on Mac OSX 10.10.4, with a 64-bit core2duo processor. Below are all 
>> 64-bit python
>> implementations:
>> 2.6.9 (apple supplied), 2.7.6 (apple supplied), 3.4.3 (homebrew), and 
>> pypy-2.6.0
>> (homebrew). I don't have any 32 bit Python implementations on the mac.
> 
> Sure you do!
> 
> $ /usr/bin/python2.7 -c 'import sys;print(sys.maxsize)'
> 9223372036854775807
> $ arch -i386 /usr/bin/python2.7 -c 'import sys;print(sys.maxsize)'
> 2147483647
> 

Woops! Totally forgot about the multiarch thing. Thanks for reminding me.
It doesn't change the outcome of the test though, when running the little test code with
the 32-bit versions they both (2.6 and 2.7) ran to completion.

Irmen

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#93450

Steven D'Aprano wrote:

> Despite the title, this is not one of the usual "Why can't Python do
> maths?" "bug" reports.
> 
> Can anyone reproduce this behaviour? If so, please reply with the version
> of Python and your operating system. Printing sys.version will probably
> do.
<zip>

On Kubuntu 15.04, it works.

Python 3.4.3 (default, Mar 26 2015, 22:03:40) 
[GCC 4.9.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> x = 1 - 1/2**53
>>> assert x == 0.9999999999999999
>>> for i in range(1, 1000000):
...     if int(i*x) == i:
...         print(i); break
... 
>>> 

A+
Laurent.

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