Can global variable be passed into Python function?

Mark Lawrence breamoreboy at yahoo.co.uk
Sun Mar 2 14:04:17 CET 2014


On 02/03/2014 09:35, Marko Rauhamaa wrote:
> Steven D'Aprano <steve+comp.lang.python at pearwood.info>:
>
>> On Sat, 01 Mar 2014 19:29:41 +0200, Marko Rauhamaa wrote:
>>> Michael Torrie <torriem at 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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