| msg102245 - (view) |
Author: Mark Dickinson (mark.dickinson) *  |
Date: 2010-04-03 11:51 |
| In Python 2.7, struct.pack with an integer format can handle non-integers that provide an __int__ method (although this *does* raise a DeprecationWarning). Python 2.7a4+ (trunk:79659:79661, Apr 3 2010, 11:28:19) [GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> from struct import pack [35194 refs] >>> pack('L', 3.1415) '\x03\x00\x00\x00\x00\x00\x00\x00' [35210 refs] This behaviour isn't particularly desirable for floats or Decimal instances, but it's useful for integer-like objects. In Python 3.x, there's no provision for handling integer-like objects than aren't actually integers. I propose that in 3.x, struct.pack should try to convert any non-integer to an integer by using its __index__ method, before packing. |
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| msg102257 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2010-04-03 13:24 |
| Here's a patch for trunk. It combines the docs and tests from Meador Inge's patch in issue 1530559 with a C-level change to get_pylong in Modules/struct.c. |
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| msg102258 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2010-04-03 13:25 |
| Adding Meador Inge to nosy. |
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| msg102262 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2010-04-03 13:57 |
| That patch was a bit hasty in many respects; here's a better one. For 2.7, the scheme is as follows: when packing a non-integer with an integer format: (1) First __index__ is tried (2) If the __index__ method doesn't exist, or the call to __index__ raises TypeError, then the __int__ method is tried. (3) If the __index__ method raises something other than TypeError, or returns a non-integer, then struct.pack fails. |
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| msg102263 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2010-04-03 14:06 |
| Committed this patch to trunk in r79674. Will forward port to py3k. |
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| msg102311 - (view) |
Author: Meador Inge (meador.inge) * |
Date: 2010-04-04 00:38 |
| I may be missing something subtle, but how can 'PyNumber_Index(v) != NULL' *and* '!PyInt_Check(v) && !PyLong_Check(v)' both be satisfied in the 'get_pylong' mods? It seems to me that 'PyNumber_Index' only returns non-NULL when the object being returned is an 'int' or 'long'. Attached a patch with the extra check removed and a few more test cases. |
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| msg102325 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2010-04-04 08:34 |
| Probably both those conditions can't be satisfied; I'm wasn't sure what happened if something's __index__ method returned something other than an int or long. But now I bother to look at the source (in Objects/abstract.c) I see that there *is* already an explicit check for the result of nb_index being int or long (with TypeError being raised if the result isn't one of those). Mea culpa. I'll remove those lines (though I may leave an assert, just to be on the safe side). The 2.x behaviour isn't ideal: I'd prefer to just stop if the __index__ method is present and raises TypeError, rather than going on to check __int__ in that case. But that presents problems with old-style classes, where PyIndex_Check is true even when no __index__ method is explicitly defined. Thanks for the extra tests! |
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| msg102326 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2010-04-04 08:44 |
| Committed (with some tabs in test_struct.py changed to spaces) to trunk in r79745. |
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| msg102327 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2010-04-04 08:53 |
| Merged to py3k in r79746. Meador, does this all look okay, now? |
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| msg102361 - (view) |
Author: Meador Inge (meador.inge) * |
Date: 2010-04-05 03:16 |
| Looks good to me. |
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| msg102364 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2010-04-05 07:47 |
| Thanks. |
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| msg128849 - (view) |
Author: Matt Joiner (anacrolix) |
Date: 2011-02-19 15:30 |
| Why isn't this implemented to work with __int__ as well? |
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| msg128877 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2011-02-19 19:07 |
| Because (arguably) we don't want to be able to pack non-integral floats (or Decimal instances, or ...) using integer formats: >>> import struct [56090 refs] >>> struct.pack('L', 2.3) Traceback (most recent call last): File "", line 1, in struct.error: required argument is not an integer [56125 refs] |
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| msg128891 - (view) |
Author: Matt Joiner (anacrolix) |
Date: 2011-02-20 04:14 |
| Thanks Mark for clearing that up. I found this link to be useful in explaining the purpose of __index__: http://docs.python.org/release/2.5.1/whatsnew/pep-357.html I think the choice of allowing only __index__ was the right choice. |
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