Another reason to avoid using
__slots__ in your Python classes
Presented in the traditional illustrated form:
class A(object): __slots__ = ('a',) class B1(A): __slots__ = ('b1',) class B2(A): __slots__ = ('b2',)
Now try to define a class C that inherits from both B1 and B2, and you will get:
TypeError: Error when calling the metaclass bases
multiple bases have instance lay-out conflict
It's relatively intuitive to see why and how this conflict happens at
an abstract level. Imagine that
__slots__ fields are sort of like
function local variables and go in an indexed array. Then class A uses
the first array slot for
a, and class B1 and B2 both use the second
array slot for
b2 respectively. When you get to C, you have a
conflict; the second array slot is used twice.
It is apparently popular in certain circles to use
to make it so that class instances can't have extra attributes added to
them at runtime. The example here illustrates the problem with this;
__slots__ has effects on what you or other people can do later
with the classes.
Sidebar: about those variable names
Update: I am wrong about what I wrote below, because it turns out
that a little knowledge is a dangerous thing. Per the Python data
model documentation on slots, a class
__slots__ definition turns off slots entirely, so
the example below isn't doing what I think it's doing. Oops.
As it turns out, you can't avoid this conflict by giving the instance variable the same name in B1 and B2. If it really is the same variable, you need to introduce a parent B class that defines it:
class B(A): __slots__ = ('b',) class B1(B): pass class B2(B): pass
Then you can define a class C that inherits from both B1 and B2.
Comments on this page:Written on 19 April 2011.