seems like a bug in isinstance()

[dmitrey]

On May 6, 12:57 pm, Chris Rebert <c... at rebertia.com> wrote:
> On Fri, May 6, 2011 at 2:24 AM, dmitrey <dmitre... at gmail.com> wrote:
> > hi all,
>
> > suppose I've created a class Point in file .../openopt/kernel/Point.py
>
> > Consider the code in file .../somewhere/file1.py
> > from openopt.kernel.Point import Point
> > p = Point()
>
> > now let's pass p into a func from .../openopt/kernel/file2.py and
> > check
> > from Point import Point
> > isinstance(p, Point)
>
> > So, it returns False!
>
> > p is <Point.Point instance at 0x30801b8>, while Point is <class
> > openopt.kernel.Point.Point at 0x2048e20>
>
> > [Subject: seems like a bug in isinstance()]
>
> This is due to a peculiarity of how (C)Python's import machinery
> works; isinstance() is working just fine.
> (And if you ever think you've found a bug in Python's built-ins, odds
> are you haven't. Python has been around too long, someone ought to
> have encountered it earlier, statistically speaking.)
>
> Note how the class is referred to as both Point.Point and
> openopt.kernel.Point.Point. This is because you did `from Point import
> ...` in file2.py, whereas in file1.py you did `from
> openopt.kernel.Point import ...`. These 2 different ways of referring
> to the same module are sufficient to "trick"/"outsmart" (C)Python and
> cause it to import the same module twice as 2 separate instances (i.e.
> it gets re-executed). Why the import machinery isn't smarter in this
> situation, I don't recall.
>
> The output of this should be illuminating:
> print(Point, type(p), type(p) is Point, id(Point), id(type(p)))
> As this demonstrates, you're dealing with 2 separate definitions of
> the same Point class.
>
> Solution: Avoid the implicitly-relative `from Point import ...` style
> of import; always use the absolute `from openopt.kernel.Point import
> ...` style instead. Subsequent imports will thus reference the
> already-previously-imported instance of a module rather than importing
> a copy of it from scratch again.
>
> Cheers,
> Chris
> --http://rebertia.com

Thanks Cris, however, I had understood reason of the bug and mere
informed Python developers of the bug to fix it.

>>> print(Point, type(p), type(p) is Point, id(Point), id(type(p)))
(<class openopt.kernel.Point.Point at 0x2a29d50>, <type 'instance'>,
False, 44211536, 8585344)

The proposed solution of using `from openopt.kernel.Point import ... '
everywhere is already performed but is not nice for me. I have lots of
places like that in my code; also, when I import something from
openopt it performs recursive import of many files including that one
where Point is defined, thus I have cycled imports (well, it somehow
works, but is unstable and may lead to future bugs). Also, writing
"from Point import Point" is much simpler than using each time the
long string "from name1.name2.Point import Point". I think it's Python
developers who have to fix the issue, not users. I have 5 years of
intensive Python experience yet it took same time to me to locate the
bug, because my algorithm got "False" from isinstance() and went
absolutely different thread from "if isinstance(): ... else: ...".
This bug could be encountered very seldom under rare circumstances and
thus be quite difficult to be located, especially for Python newbies
unawared of this one.

Regards, D.