FAQ

On 8/26/2015 5:21 PM, Rob Gaddi wrote:
I'm running into some strangeness trying to work with the bitfield module
from my ctypes-bitfield package (on PyPi). I'm trying to use isinstance
(), and it's kinda sorta lying to me.

isinstace(inst, klass) is implemented as
klass.__instancecheck__(inst) ==
type(klass).__instancecheck__(klass, inst)
In other words, __instancecheck__ is a metaclass method.
Reference Manual, 3.3.4. Customizing instance and subclass checks

----- IPython session (Python 3.4 under Linux) -------
In [649]: bf.__mro__
Out[649]: (bitfield._TD, _ctypes.Union, _ctypes._CData,
bitfield.Bitfield, builtins.object)

Find the metaclasses with "for cl in bf.__mro__: print(type(bf))" and
then take a look at the __instancecheck__ method if not 'type'.

In [650]: isinstance(bf, bitfield.Bitfield)
Out[650]: False

In [651]: bf.__bases__
Out[651]: (_ctypes.Union, bitfield.Bitfield)

In [652]: bf.__bases__[1]
Out[652]: bitfield.Bitfield

In [653]: bf.__bases__[1] is bitfield.Bitfield
Out[653]: True
-------------------------------------------------------

Is there an issue with isinstance and multiple inheritance? Conversely
is there an issue with isinstance and ctypes derived classes (Bitfield
isn't, but Bitfield is a mixin that always works with Unions) I know
that ctypes classes can get really wonky on this stuff.

More generally, is there any good way to introspect ctypes derived
classes? I have to figure out whether things are derived from Structure,
Union, Array etc. through some ugly indirect methods, and have no idea
why.



--
Terry Jan Reedy

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postedAug 26, '15 at 9:21p
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