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I just came across this (https://gist.github.com/1208215):

import sys
import ctypes
pyint_p = ctypes.POINTER(ctypes.c_byte*sys.getsizeof(5))
five = ctypes.cast(id(5), pyint_p)
print(2 + 2 == 5) # False
five.contents[five.contents[:].index(5)] = 4
print(2 + 2 == 5) # True (must be sufficiently large values of 2 there...)

Heh. The author is apparently anonymous, I guess for good reason.

Paul Rubin, 04.07.2012 21:37:
> I just came across this (https://gist.github.com/1208215):
>
> import sys
> import ctypes
> pyint_p = ctypes.POINTER(ctypes.c_byte*sys.getsizeof(5))
> five = ctypes.cast(id(5), pyint_p)
> print(2 + 2 == 5) # False
> five.contents[five.contents[:].index(5)] = 4
> print(2 + 2 == 5) # True (must be sufficiently large values of 2 there...)
>
> Heh. The author is apparently anonymous, I guess for good reason.

That's not portable, though.

Stefan

Am 04.07.2012, 21:37 Uhr, schrieb Paul Rubin <phr->:

> I just came across this (https://gist.github.com/1208215):
>
> import sys
> import ctypes
> pyint_p = ctypes.POINTER(ctypes.c_byte*sys.getsizeof(5))
> five = ctypes.cast(id(5), pyint_p)
> print(2 + 2 == 5) # False
> five.contents[five.contents[:].index(5)] = 4
> print(2 + 2 == 5) # True (must be sufficiently large values of 2
> there...)
>
> Heh. The author is apparently anonymous, I guess for good reason.


Neat.

Playing with it, i'm wondering:


This:

import sys
import ctypes
pyint_p = ctypes.POINTER(ctypes.c_byte*sys.getsizeof(5))
five = ctypes.cast(id(5), pyint_p)
five.contents[five.contents[:].index(5)] = 4

print ( 2 + 2 == 5 )
print 5
print 5 - 2

put into a script and run prints:

True
4
3

while entered at the python prompt it prints:

True
4
2

??


Regards,
Michael

On 04/07/2012 20:37, Paul Rubin wrote:
> I just came across this (https://gist.github.com/1208215):
>
> import sys
> import ctypes
> pyint_p = ctypes.POINTER(ctypes.c_byte*sys.getsizeof(5))
> five = ctypes.cast(id(5), pyint_p)
> print(2 + 2 == 5) # False
> five.contents[five.contents[:].index(5)] = 4
> print(2 + 2 == 5) # True (must be sufficiently large values of 2 there...)
>
> Heh. The author is apparently anonymous, I guess for good reason.
>

The author got confused trying to switch from imperial to metric numbers
or vice versa?

--
Cheers.

Mark Lawrence.

On 07/04/2012 09:37 PM, Paul Rubin wrote:
> I just came across this (https://gist.github.com/1208215):
>
> import sys
> import ctypes
> pyint_p = ctypes.POINTER(ctypes.c_byte*sys.getsizeof(5))
> five = ctypes.cast(id(5), pyint_p)
> print(2 + 2 == 5) # False
> five.contents[five.contents[:].index(5)] = 4
> print(2 + 2 == 5) # True (must be sufficiently large values of 2 there...)
>
> Heh. The author is apparently anonymous, I guess for good reason.
>

I'm reminded of the swap(a,b) function I wrote a couple of years back.

http://blog.jollybox.de/archives/62-python-swap

On Jul 5, 5:37*am, Paul Rubin <phr-2...@nightsong.com> wrote:
> * * print(2 + 2 == 5) # True (must be sufficiently large values of 2 there...)

I believe CCP use a variant of this approach in EvE with Stackless
Python.

On 7/4/2012 14:37, Paul Rubin wrote:
> I just came across this (https://gist.github.com/1208215):
>
> import sys
> import ctypes
> pyint_p = ctypes.POINTER(ctypes.c_byte*sys.getsizeof(5))
> five = ctypes.cast(id(5), pyint_p)
> print(2 + 2 == 5) # False
> five.contents[five.contents[:].index(5)] = 4
> print(2 + 2 == 5) # True (must be sufficiently large values of 2 there...)
>
> Heh. The author is apparently anonymous, I guess for good reason.

Probably just nostalgic for old Fortran, which, supposedly, allowed you
to change the values of literals by passing them to a function by
reference and then modifying the value.

Evan



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On 7/4/2012 4:37 PM, Michael Ross wrote:
> Am 04.07.2012, 21:37 Uhr, schrieb Paul Rubin <phr->:
>
>> I just came across this (https://gist.github.com/1208215):
>>
>> import sys
>> import ctypes
>> pyint_p = ctypes.POINTER(ctypes.c_byte*sys.getsizeof(5))
>> five = ctypes.cast(id(5), pyint_p)
>> print(2 + 2 == 5) # False
>> five.contents[five.contents[:].index(5)] = 4
>> print(2 + 2 == 5) # True (must be sufficiently large values of 2
>> there...)
>>
>> Heh. The author is apparently anonymous, I guess for good reason.
>
>
> Neat.
>
> Playing with it, i'm wondering:
>
>
> This:
>
> import sys
> import ctypes
> pyint_p = ctypes.POINTER(ctypes.c_byte*sys.getsizeof(5))
> five = ctypes.cast(id(5), pyint_p)
> five.contents[five.contents[:].index(5)] = 4
>
> print ( 2 + 2 == 5 )
> print 5
> print 5 - 2
>
> put into a script and run prints:
>
> True
> 4
> 3

The compile-time optimizer computed the contant 5-2 in C.

> while entered at the python prompt it prints:
>
> True
> 4
> 2

It must not run for interactive input with the undisclosed version you
are running.

If I run the script in 3.3 Idle, I get the same output you got. If I
then enter '5-2' interactively, I still get 3. Maybe the constant folder
is always on now.

--
Terry Jan Reedy

On Wed, 04 Jul 2012 23:38:17 -0400, Terry Reedy wrote:

> If I run the script in 3.3 Idle, I get the same output you got. If I
> then enter '5-2' interactively, I still get 3. Maybe the constant folder
> is always on now.

Yes, I believe constant folding is always on, since Python 2.4 if I
remember correctly. Somebody who cares more than me can possibly check
the "What's New" documents


--
Steven

On 5/07/12 07:32:48, Steven D'Aprano wrote:
> On Wed, 04 Jul 2012 23:38:17 -0400, Terry Reedy wrote:
>
>> If I run the script in 3.3 Idle, I get the same output you got. If I
>> then enter '5-2' interactively, I still get 3. Maybe the constant folder
>> is always on now.
>
> Yes, I believe constant folding is always on, since Python 2.4 if I
> remember correctly. Somebody who cares more than me can possibly check
> the "What's New" documents

It's not a difference between 2.4 and 3.3; the difference is between
Idle and the command-line version of the interactive interpreter.

If I type the same code into Idle and the interactive interpreter
(both using 3.3alpha1), I get 3 in Idle and 2 in Terminal.

I don't quite understand why this difference exists.


Confused,

-- HansM