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 -- https://mail.python.org/mailman/listinfo/python-list
