Re: [Python-Dev] struct module and coercing floats to integers
On Jul 28, 2006, at 1:35 PM, Bob Ippolito wrote:
It seems that the pre-2.5 struct module has some additional
undocumented behavior[1] that didn't percolate into the new version:
http://python.org/sf/1530559
Python 2.4 and previous will coerce floats to integers when necessary
as such without any kind of complaint:
$ python2.4 -c import struct; print repr(struct.pack('H',
0.))
'\x00\x00'
Python 2.5 refuses to coerce float to int:
$ python2.5 -c import struct; print repr(struct.pack('H',
0.))
Traceback (most recent call last):
File string, line 1, in module
File /Users/bob/src/python/Lib/struct.py, line 63, in pack
return o.pack(*args)
TypeError: unsupported operand type(s) for : 'float' and 'long'
The available options are to:
1. Reinstate the pre-2.5 weirdness
2. Reinstate the pre-2.5 weirdness with a DeprecationWarning
3. Break existing code that relies on undocumented behavior (seems
more like a bug than lack of specification)
There's a patch in the tracker for 2. It should get applied when the
trunk freeze is over.
-bob
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[Python-Dev] struct module and coercing floats to integers
It seems that the pre-2.5 struct module has some additional
undocumented behavior[1] that didn't percolate into the new version:
http://python.org/sf/1530559
Python 2.4 and previous will coerce floats to integers when necessary
as such without any kind of complaint:
$ python2.4 -c import struct; print repr(struct.pack('H',
0.))
'\x00\x00'
Python 2.5 refuses to coerce float to int:
$ python2.5 -c import struct; print repr(struct.pack('H',
0.))
Traceback (most recent call last):
File string, line 1, in module
File /Users/bob/src/python/Lib/struct.py, line 63, in pack
return o.pack(*args)
TypeError: unsupported operand type(s) for : 'float' and 'long'
The available options are to:
1. Reinstate the pre-2.5 weirdness
2. Reinstate the pre-2.5 weirdness with a DeprecationWarning
3. Break existing code that relies on undocumented behavior (seems
more like a bug than lack of specification)
Either 2 or 3 seems reasonable to me, with a preference for 3 because
none of my code depends on old bugs in the struct module :)
As far as precedent goes, the array module *used* to coerce floats
silently, but it's had a DeprecationWarning since at least Python 2.3
(but perhaps even earlier). Maybe it's time to promote that warning
to an exception for Python 2.5?
[1] The pre-2.5 behavior should really be considered a bug, the
documentation says Return a string containing the values v1, v2, ...
packed according to the given format. The arguments must match the
values required by the format exactly. I wouldn't consider arbitrary
floating point numbers to match the value required by an integer
format exactly. Floats are not in general interchangeable with
integers in Python anyway (e.g. list indexes, etc.).
-bob
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Re: [Python-Dev] struct module and coercing floats to integers
On 7/28/06, Bob Ippolito [EMAIL PROTECTED] wrote:
http://python.org/sf/1530559
[1] The pre-2.5 behavior should really be considered a bug, the
documentation says Return a string containing the values v1, v2, ...
packed according to the given format. The arguments must match the
values required by the format exactly. I wouldn't consider arbitrary
floating point numbers to match the value required by an integer
format exactly. Floats are not in general interchangeable with
integers in Python anyway (e.g. list indexes, etc.).
While it may be a bug, it's not as hard to run into, nor as illogical
as the presentation here makes it sound. The original code[1] took a
float value between 0 and 2, and wanted to use
pack('H', round(value * 32768))
The workaround is a trivial change
pack('H', int(round(value * 32768)))
but the timeframe is less than ideal, as working code will suddenly
stop and recieve only mildly helpful error message. The fact that
round returns a float rather than an int, while intentional, does not
feature prominently in one's mine when the first version yielded the
expected results.
I would appreciate option 2 which retains compatibility but warns that
the construct is bad. I will accept any of the options, as it's clear
that floats don't make sense. It's just unfortunate that the previous
implementation let them through in a way the new implementation does
not.
[1] http://www.sacredchao.net/quodlibet/changeset/3706
Michael
--
Michael Urman http://www.tortall.net/mu/blog
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