[Python-Dev] Should issubclass() be more like isinstance()?

[Alex Martelli]

On Apr 4, 2006, at 8:18 PM, Crutcher Dunnavant wrote:
    ...
>> There's no rule that predicate cannot raise an exception.
>
> No, but it makes many applications (such as using it as a test in list
> comprehensions) difficult enough to not be worth it.

IMHO, the solution to THAT very real problem is to supply a built-in  
function that catches some exceptions and maps them into suitable  
return values. Simplest would be something like:

def catch(excepts, default, f, *a, **k):
     try:
         return f(*a, **k)
     except excepts:
         return default

and then, the LC you're after is easy:

subints = [x for x in y if catch(TypeError, False, issubclass, x, int)]

though I'm sure we can get better syntax if we turn 'catch' into some  
kind of syntactic special form.  My point is that there are umpteen  
predicates one can write which would have to be distorted to ensure  
they can't raise -- better to let them raise if they must, and allow  
catching the expected exception(s), somewhat like this example.


Alex