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[Python-Dev] How long the wrong type of argument should we limit (or not) in the error message (C-api)?

Nick Coghlan ncoghlan at gmail.com
Sun Dec 15 22:30:56 CET 2013 
On 16 Dec 2013 02:58, "Ethan Furman" <ethan at stoneleaf.us> wrote:
>
> On 12/14/2013 07:51 PM, Steven D'Aprano wrote:
>>
>> On Sun, Dec 15, 2013 at 11:25:10AM +1000, Nick Coghlan wrote:
>>
>>> Oh, yes, a %T shortcut for "length limited type name of the supplied
>>> object" would be brilliant. We need this frequently for C level error
>>> messages, and I almost always have to look at an existing example to
>>> remember the exact incantation :)
>>
>>
>> What are the chances that could be made available from pure Python too?
>> Having to extract the name of the type is a very common need for error
>> messages, and I never know whether I ought to write type(obj).__name__
>> or obj.__class__.__name__. A %T and/or {:T} format code could be the One
>> Obvious Way to include the type name in strings
>
>
> +1

It's less obviously correct for Python code, though. In C, we're almost
always running off slots, so type(obj).__name__ has a very high chance of
being what we want, and is also preferred for speed reasons (since it's
just a couple of pointer dereferences).

At the Python level, whether to display obj.__name__ (working with a class
directly), type(obj).__name__ (working with the concrete type, ignoring any
proxying) or obj.__class__.__name__ (which takes proxying into account)
really depends on exactly what you're doing, and the speed differences
between them aren't so stark.

Cheers,
Nick.

>
> --
> ~Ethan~
>
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