Python - "object layout"

Question

can somebody describe the following exception? What is the "object layout" and how it is defined? Thanks

Traceback (most recent call last):
  File "test_gui.py", line 5, in <module>
    suite = AlgorithmEngine('gui_suite')
  File "/home/honza/Research/Voiar/algorithm.py", line 169, in __init__
    self.algorithms = self._initAlgorithms()
  File "/home/honza/Research/Voiar/algorithm.py", line 232, in _initAlgorithms
    self._initGUIAlgorithm(obj)
  File "/home/honza/Research/Voiar/algorithm.py", line 218, in _initGUIAlgorithm
    cls.__bases__ = bases
TypeError: __bases__ assignment: 'QWidget' object layout differs from 'GUIAlgorithm'

Clouet

Possibly related: autoreload gratuitous "object layout differs" error – Pedro

Answer 1

It means that you tried to change the type of an object (by assigning to __bases__) and the new type wasn't compatible with the old one. This happens when the underlying C data structure isn't the same for both types.

See http://www.mail-archive.com/[email protected]/msg52950.html for a list of differences between types that might trigger this exception.

Inexhaustible

It is slightly disappointing how many caveats in general one runs into when going into the (exposed) internals of Python--the consistency and cleanness of Python just breaks down there. This here is just one example. – Wildon

@ErikKaplun replacing __bases__ on the fly is hackery and cannot therefore contribute to the overall impression of a language :) – Gilbertogilbertson

Gonna add going from a pure Python class at the start to adding in collections.abc.Mapping in the hierarchy to assist with making my original __class__ behave more like a dict. Assigning to self.__class__ (again a pure Python class) then give this exact error. Take away subclassing collections.abc.Mapping and error goes away. – Volgograd

Answer 2

In my case, the error occurred when I tried to use change the __class__ of an object that also has __slots__, like this:

class Base:
    __slots__ = ('a', 'b', 'c')

class Child(Base):
    pass


obj = Base()
obj.__class__ = Child  
# -> TypeError: __class__ assignment: 'Child' object layout differs from 'Base'

Gilbertogilbertson

Child should have __slots__ = tuple() – Sennar