Can global variable be passed into Python function?

Steven D'Aprano steve+comp.lang.python at pearwood.info
Sun Mar 2 11:44:19 CET 2014


On Sun, 02 Mar 2014 11:35:43 +0200, Marko Rauhamaa wrote:

> 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.

Um, yes? Nor can you use ">" to test for identity, or "+", or "%", or any 
other operator other than "is". Why do you think it is a problem that == 
doesn't test for identity? It's not supposed to test for identity.

Why do you want to test for identity? I think I've asked five times now, 
why you care whether a state value has one instance or a thousand 
instances, and you haven't even attempted an answer.



> 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.

It means that a equals b. For ints, it means that they have the same 
numeric value. The same applies for floats. For strings, it means that 
they contain the same code points in the same order. And so on. For all 
built-in types, equality is well-defined. For custom types you create 
yourself, the onus is on you to ensure that equality is meaningful and 
well-defined.


> 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.

Incorrect. We are supposed to know and care.

os.posix is exactly the sort of library I mentioned earlier when I said 
sometimes you're constrained by compatibility with some other system. In 
this case, the os module is explicitly designed to be compatible with the 
POSIX interface, which is defined to use certain integer values as flags. 
This is not an implementation choice which implementers can change at 
will, it is part of the interface.

The specific *values* possibly may be allowed to vary from platform to 
platform, and since this is C even the definition of "int" may be 
platform specific, but not that fact that they are ints. Hence the value 
of POSIX_FADV_RANDOM could, theoretically, be different under Linux and 
FreeBSD (say). It probably isn't, but it could be. If you hard-code the 
magic number 1 in your code, you're risking the (tiny) chance of it 
failing on some obscure POSIX system. But that doesn't imply that we must 
test for object identity. There could be a million different instances, 
all with the value POSIX_FADV_RANDOM.

Python does not guarantee that there is only a single 1 instance. If you 
want to test whether a value is os.POSIX_FADV_RANDOM, the right way is to 
compare that value for equality with os.POSIX_FADV_RANDOM, not identity.


> 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

A much better choice than hard-coding the magic value 1. But that choice 
has absolutely nothing to do with whether 1 is a singleton or not.


> in some other part of my code I might want to see how "flag" was set.
> Should I use "==" or "is" to test it?

Equals, of course. There is absolutely no question about that. To even 
*think* that you should test it with "is" means that you have completely 
misunderstood what you are doing here. Why are you relying on an 
implementation detail that CPython happens to cache and reuse small 
integers like 1? What happens if you run your code under an 
implementation of Python that doesn't cache small ints? Or if your 
platform happens to set POSIX_FADV_RANDOM to a non-cached value like 
8531201?

Python does not promise that POSIX_FADV_RANDOM will be a singleton value. 
Using "is" is unsafe.


> If I take the API documentation on its face value, I *must* use "==" for
> os.SEEK*:

Correct.


>     if seek_flag == os.SEEK_END:
>         ...
> 
> and I *must* use "is" for os.POSIX_FAVD_*:

Incorrect.


>     if fsavd_flag is os.POSIX_FADV_RANDOM:
>         ...
> 
> Since, for all I know, os.POSIX_FAVD_RANDOM might return a random value
> for __eq__().

For all *you* know, perhaps, but since os.posix_fadvise is a thin wrapper 
around the POSIX C function fadvise, and that is documented as expecting 
ints for the advice parameter, that cannot be the case.

Unfortunately Python has not had the money put into it to make it an ISO 
standard like Java, and so there are certain areas where the behaviour is 
known by common practice but not officially documented. (A bit like 
British common law.) That the os module is a thin wrapper around os-
specific services may not be explicitly stated, but it is nevertheless 
true.



-- 
Steven D'Aprano
http://import-that.dreamwidth.org/



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