On 02/03/2014 09:35, Marko Rauhamaa wrote:
Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info>:
On Sat, 01 Mar 2014 19:29:41 +0200, Marko Rauhamaa wrote:
Michael Torrie <torr...@gmail.com>:
No, '==' works fine no matter what objects you assign to your state
variables.
Well, it doesn't since
>>> a = float("nan")
>>> a is a
True
>>> a == a
False
No, that is working correctly, so the comment that equals works fine
is correct: returning False is the correct thing to do if one or both
of the objects are a NAN. NANs are supposed to compare unequal to
everything, including themselves.
Nobody is saying there's a bug in the implementation of "==". I'm just
saying "==" cannot be taken as a universal superset of "is". Therefore
a program cannot blindly use "==" to test for identity.
That's why "==" is a bit fishy. It immediately raises the question: what
does it mean for a == b, especially since the exact implementation of a
and b are intended to be opaque.
Example:
The os module defines the constants os.SEEK_SET, os.SEEK_CUR and
os.SEEK_END that can be used as arguments for os.lseek(). Must those
constants be used, or can a regular integer be used instead? The
documentation clearly states that integers can be used:
SEEK_SET or 0 to set the position relative to the beginning of the
file; SEEK_CUR or 1 to set it relative to the current position;
SEEK_END or 2 to set it relative to the end of the file.
However, on the same reference page, os.posix_fadvise() is defined. We
read:
advice is one of POSIX_FADV_NORMAL, POSIX_FADV_SEQUENTIAL,
POSIX_FADV_RANDOM, POSIX_FADV_NOREUSE, POSIX_FADV_WILLNEED or
POSIX_FADV_DONTNEED
and:
os.POSIX_FADV_NORMAL
os.POSIX_FADV_SEQUENTIAL
os.POSIX_FADV_RANDOM
os.POSIX_FADV_NOREUSE
os.POSIX_FADV_WILLNEED
os.POSIX_FADV_DONTNEED
Flags that can be used in advice in posix_fadvise()
Now, what kinds of object are those constants? We are not supposed to
know or care. We could peek into the implementation, but it would be a
grave mistake to trust the implementation choices in the application.
So in my application code I might set:
favd_flag = os.POSIX_FADV_RANDOM
in some other part of my code I might want to see how "flag" was set.
Should I use "==" or "is" to test it?
If I take the API documentation on its face value, I *must* use "==" for
os.SEEK*:
if seek_flag == os.SEEK_END:
...
and I *must* use "is" for os.POSIX_FAVD_*:
if fsavd_flag is os.POSIX_FADV_RANDOM:
...
Since, for all I know, os.POSIX_FAVD_RANDOM might return a random value
for __eq__().
Marko
Will you please be kind enough to stop writing this drivel. You've been
told repeatedly that you don't know what you're talking about, so have
the decency to just belt up. Now try testing these words for identity.
--
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.
Mark Lawrence
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