Home Original page

Behaviour-based interface/protocol implementation?

Terry Reedy tjreedy at udel.edu
Tue Jan 25 12:48:15 EST 2011 
On 1/25/2011 10:32 AM, Alan Franzoni wrote:

> You're right, I forgot about subclass check. But that's really a
> placebo, because it statically checks the object's *class* for such
> method,

That is exactly the proper check. Instance *methods* are defined on the 
class.

 > not the actual instance:

Function attributes of instances are not methods. This is especially 
true of special methods.

> from collections import Sized
>
> class MyObj(object):
>      pass
>
> mo = MyObj()
> mo.__len__ = lambda: 1
>
>
> print isinstance(mo, Sized)
> False

This is correct!

print(len(mo))
TypeError: object of type 'MyObj' has no len()

-- 
Terry Jan Reedy
More information about the Python-list mailing list