Steven D'Aprano steve+comp.lang.python at
Tue Feb 5 07:31:22 CET 2013

On Tue, 05 Feb 2013 16:20:19 +1100, Chris Angelico wrote:

> On Tue, Feb 5, 2013 at 3:52 PM, Steven D'Aprano
> <steve+comp.lang.python at> wrote:
>> There's also the principle that it is best to raise an exception as
>> early as possible. It's easier to track down errors at the point they
>> are introduced than long afterwards.
> Yes, definitely, especially (as was mentioned) if you're working with
> callbacks. But I'd use isinstance then.

I'm leaning towards an isinstance check

I've been using Python since Python 1.5. Even though 1.5 had an 
"isinstance" function, I learned Python from books and code written for 
Python 1.4 which did not have that function, so the usual way to do type-
checking was:

if type(obj) is type(1):
    # it's an int

and strongly discouraged. So my instincts are still very strongly primed 
to *not* do type-checking if I can avoid it. But in this case, I think I 
agree with those suggesting the isinstance check is the right approach. 
The main downside to this is that objects which delegate to a number will 
not work unless they are explicitly registered with the Number ABC.

Thanks to everyone who responded.


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