| msg64037 - (view) |
Author: Terry J. Reedy (terry.reedy) *  |
Date: 2008-03-19 04:45 |
| Python 3.0a3 (r30a3:61161, Mar 1 2008, 22:51:17) [MSC v.1500 32 bit (Intel)] on win32 >>> a,b=1,1//1 >>> a is b False IDLE 3.0a3 >>> a,b=1,1//1 >>> a is b True ditto for 2.5.2 interpreter On c.l.p, Duncan Booth wrote I've had a look to see why this happens: long division (and in Python 3 all integers are longs) allocates a new long to hold the result of the division so it will never use one of the preallocated 'small int' values. That maybe explains the change from 2.5 but not the difference from IDLE. More important, the small int checks are present with the other operations: >>> 1*1 is 1 True >>> 1+1 is 2 True >>> 1-1 is 0 True >>> 1**1 is 1 True so the omission with // is plausibly a bug. |
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| msg64060 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2008-03-19 15:13 |
| I agree this is a bug. Here is a related problem: >>> 1 is divmod(1,1)[0] False |
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| msg64063 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2008-03-19 15:49 |
| >>> int('1') is 1 False >>> 1 is int('0b1', 2) False >>> 1 is 0b10000 >> 4 False there are probably more ... |
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| msg64064 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2008-03-19 15:57 |
| >>> 1 is 1|1 False >>> 1 is 1&1 False >>> 0 is 1^1 False >>> 2 is 1<<1 False |
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| msg64065 - (view) |
Author: Facundo Batista (facundobatista) * |
Date: 2008-03-19 16:06 |
| I really don't understand *why* it's a bug. Is anywhere in the Language Reference that says that 1 should be the same object that another 1? Or do you mean that they *should* be the same object for speed and/or memory reasons? In this case... is this a bug? Or a request for improvement or something? |
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| msg64066 - (view) |
Author: Adam Olsen (Rhamphoryncus) |
Date: 2008-03-19 16:19 |
| The original bug is not whether or not python reuses int objects, but rather that an existing optimization disappears under certain circumstances. Something is breaking our optimization. The later cases where the optimization is simply gone in 3.0a3 are more difficult. We don't *have* to keep them.. but we probably want them. The small int reuse happens for a reason. |
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| msg64090 - (view) |
Author: Terry J. Reedy (terry.reedy) * |
Date: 2008-03-19 19:33 |
| I agree that this is not a bug in the strict sense. I could have selected Type: resource usage (for memory increase). But the change of behavior is also visible. I suspect the change is not intentional both because of the pattern and because of recent discussion on PyDev that small int caching should be kept. If the precise change is intentional, this should be documented (here) to answer further questions on c.l.p. |
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| msg64119 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2008-03-19 23:07 |
| Attached patch fixes the cases discovered so far. It is possible that in some of these cases creation of the long object that is later discarded can be avoided by detecting small int return early, but such optimizations should probably be left for later. Any advise on how to write a unit test? Should the unit test detect python compiled with NSMALLNEGINTS + NSMALLPOSINTS == 0 and disable identity checks? |
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| msg64122 - (view) |
Author: Adam Olsen (Rhamphoryncus) |
Date: 2008-03-19 23:25 |
| Unless someone has a legitimate use case for disabling small_int that doesn't involve debugging (which I really doubt), I'd just assume it's always in use. |
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| msg64150 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2008-03-20 04:46 |
| Attached (issue2417a.diff) patch adds unit tests and fixes a few corner cases when a non-preallocated 0 was returned to python. |
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| msg70185 - (view) |
Author: Facundo Batista (facundobatista) * |
Date: 2008-07-24 01:09 |
| Alexander, tried the issue2417a.diff patch against 65210, and does not apply cleanly, could you please submit an updated one? Thanks! |
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| msg70188 - (view) |
Author: Alexander Belopolsky (belopolsky) * |
Date: 2008-07-24 02:26 |
| It looks like e-mail submission did not work. Uploading updated patch as issue2417b.diff . |
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| msg70220 - (view) |
Author: Facundo Batista (facundobatista) * |
Date: 2008-07-24 18:57 |
| Commited in r65220. Thank you everybody!! |
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