python - Triple inheritance causes metaclass conflict... Sometimes -

looks stumbled upon metaclass hell when didn't wanted it.

i'm writing app in qt4 using pyside. want separate event-driven part ui definition, generated qt designer files. hence create "controller" classes, ease life multiple-inherit them anyways. example:

class basecontroller(qobject):     def setupevents(self, parent):         self.window = parent  class maincontroller(basecontroller):     pass  class mainwindow(qmainwindow, ui_mainwindow, maincontroller):     def __init__(self, parent=none):         super(mainwindow, self).__init__(parent)          self.setupui(self)         self.setupevents(self) 

this works expected. has inheritance (qdialog, ui_dialog, basecontroller). when subclass basecontroller , try inherit said subclass (in place of basecontroller), receive error:

typeerror: error when calling metaclass bases metaclass conflict: metaclass of derived class must (non-strict) subclass of metaclasses of bases

clarification: both qmainwindow , qdialog inherit qobject. basecontroller must inherit because of qt event system peculiarities. ui_ classes inherit simple python object class. searched solutions, of them involve cases of intentionally using metaclasses. must doing terribly wrong.

edit: description may clearer adding graphs.

working example:

qobject |      \___________________ |            object        | qmainwindow     |          basecontroller |      /---ui_mainwindow   | |      |                   maincontroller mainwindow-----------------/ 

another working example:

qobject |      \___________________ |            object        | qdialog         |          basecontroller |      /---ui_otherwindow  | |      |                   | otherwindow----------------/ 

not working example:

qobject |      \___________________ |            object        | qdialog         |          basecontroller |      /---ui_otherwindow  | |      |                   othercontroller otherwindow----------------/ 

the error message indicates have 2 conflicting metaclasses somewhere in hierarchy. need examine each of classes , qt classes figure out conflict is.

here's simple example code sets same situation:

class metaa(type):     pass class metab(type):     pass class a:     __metaclass__ = metaa class b:     __metaclass__ = metab 

we can't subclass both of classes directly, because python wouldn't know metaclass use:

>>> class broken(a, b): pass ...  traceback (most recent call last):   file "<stdin>", line 1, in <module> typeerror: error when calling metaclass bases   metaclass conflict: metaclass of derived class must (non-strict)   subclass of metaclasses of bases 

what error trying tell need resolve conflict between 2 metaclasses introducing third metaclass subclass of metaclasses base classes.

i'm not sure that's clearer error message itself, basically, fix doing this:

class metaab(metaa, metab):     pass  class fixed(a, b):     __metaclass__ = metaab 

this code compiles , runs correctly. of course, in real situation, conflict-resolving metaclass have decide of parent metaclass behaviors adopt, you'll have figure out application's requirements.

bear in mind inherited class gets one of 2 metaclass.__init__ methods, work, in lot of cases, going have add __init__ calls both in way helps them along.