A couple questions about classes and inheritance

[Steven D'Aprano]

On Sun, 16 May 2010 18:58:45 -0400, Paul LaFollette wrote:

> First, I've looked a fair bit and can't find how one can find the base
> classes of a subclass?

subclass.__base__
subclass.__bases__
subclass.__mro__

(The third one stands for "Method Resolution Order".)


See also the inspect module.



> So, it appears that function is perhaps the equivalent of a Java final
> class? Or isn't really a class at all but something that "looks like" a
> class?

No, it really is a class, at least as far as Python is concerned:

>>> type(lambda: None)
<class 'function'>
>>> type(type(lambda: None))
<class 'type'>

but as you've discovered, it's also special. Unfortunately I can't find 
any explanation of *why* (other than the obvious that subclassing doesn't 
work).



> Anyway, again can you point me to somewhere that I can learn more?  In
> particular, is there a list somewhere of the builtin types that are not
> subclassable?


I don't believe so. Apart from intellectual curiosity, is there any 
reason you need to know?


This may be helpful:

http://www.python.org/dev/peps/pep-0253/

You can also google for the error message, although in my few minutes of 
searching I've only found people demonstrating the failure and not 
explaining it.

Here's a rather long discussion about subclassing function:

http://mail.python.org/pipermail/python-list/2008-March/531661.html

but again, no explanation of why type(function) isn't subclassable.




-- 
Steven