Is 'isinstance()' the right thing?

[Ralf Juengling]

Hi,

when I want to make sure within a function that it can deal with
the arguments passed, the only choice is to check it's type via
'isinstance()'. However, in general the type of an argument is 
not the crux but whether the argument supports a certain 
'interface' or 'protocol'. Thus, a function 'hasinterface()'
would appear more natural (more 'Pythonic') to me.

So, what is the proper, 'pythonic' way of argument checking?

Regards,
Ralf