[Numpy-discussion] Python 3 and isinstance(np.int64(42), int)

Sebastian Berg sebastian at sipsolutions.net
Thu Jun 18 04:38:43 EDT 2015

In some cases calling operator.index(n) may yield the desired result. I
like operator.index, but maybe it is just me :). That uses duck typing
instead of instance checking to ask if it represents an integer.
But it also has some awkward corner cases in numpy, since arrays with a
single element (deprecation pending) and 0D arrays (will continue) say
they are integers when asked that way.

- Sebastian 

On Mi, 2015-06-17 at 23:13 -0700, Nathaniel Smith wrote:
> On Wed, Jun 17, 2015 at 10:53 PM, Jens Jørgen Mortensen
> <jensj at fysik.dtu.dk> wrote:
> >  >>> type(n)
> > <type 'numpy.int64'>
> >  >>> isinstance(n, int)
> > True
> >
> > With Python 3.4 you get False.  I think I understand why (np.int64 is no
> > longer a subclass of int).
> Yep, that's correct.
> > So, I did this instead:
> >
> > import numbers
> > isinstance(n, numbers.Integral)
> >
> > which works fine (with numpy-1.9).  Is this the "correct" way or is
> > there a better way to do it?
> That's the correct way to check whether an arbitrary object is of some
> integer-like-type, yes :-). There are alternatives, and there's some
> argument that in Python, doing explicit type checks like this is
> usually a sign that one is doing something awkward, but that's a more
> general issue that it's hard to comment on here without more detail
> about what exactly you're trying to accomplish.
> > I would imagine that a lot of code will
> > break because of this - so it would be nice if isinstance(n, int) could
> > be made to work the same way in 2 and 3, but I don't know if this is
> > possible (or desirable).
> It's not possible, unfortunately. In py2, 'int' is a 32- or 64-bit
> integer type, so we can arrange for numpy's int32 or int64 objects to
> be laid out the same in memory, so everything in python that expects
> an int (including C API functions) can handle a numpy int. In py3,
> 'int' is an arbitrary width integer bignum, like py2 'long', which is
> fundamentally different from int32 and int64 in both semantics and
> implementation.
> -n

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