On Sat, Feb 14, 2015 at 2:36 PM, Georg Brandl <g.brandl@gmx.net> wrote:

> In the case of int, there is a good reason for this behavior - bool. In python,

> we want True + True == 2. In numpy, where binary operations preserve

> subclasses, you have

>

>>>> import numpy

>>>> numpy.bool_(1) + numpy.bool_(1)

> True

I don't think numpy.bool_ subclasses some class like numpy.int_.

And numpy.bool_ subclasses don't preserve type in addition:

>>> import numpy

>>> class Bool(numpy.bool_):

... pass

...

>>> numpy.bool_.mro()

[<class 'numpy.bool_'>, <class 'numpy.generic'>, <class 'object'>]

>>> Bool(1) + Bool(1)

True

>>> type(_)

<class 'numpy.bool_'>

So there goes my theory. :-)

I think all these examples just highlight the need for a clear guidance when self.__class__() can be called in base classes to construct instances of derived classes.

Apparently numpy has it both ways. One way for scalars (see above) and the other for arrays:

>>> class Array(numpy.ndarray):

... pass

...

>>> a = Array(1)

>>> a[0] = 1

>>> a+a

Array([ 2.])