[Python-ideas] Changing str(someclass) to return only the class name

[Guido van Rossum]

On Sat, Oct 22, 2011 at 2:44 PM, Ben Finney <ben+python at benfinney.id.au> wrote:
> Guido van Rossum <guido at python.org> writes:
>
>> On Sat, Oct 22, 2011 at 1:18 PM, Steven D'Aprano <steve at pearwood.info> wrote:
>
>> > I'm just going to repeat what I've said before: explicit is better
>> > than implicit. If you want the name of an object (be it a class, a
>> > module, a function, or something else), you should explicitly ask
>> > for the name, and not rely on its str().
>
> +1. Many objects, specifically including exceptions, have something more
> useful than the name to return from 'str(x)'.
>
>> > However, the docs also say that str() should return [quote] a string
>> > containing a nicely printable representation of an object [end
>> > quote].
>
> Yes. The name of an object (if it has one) is often just one part of the
> representation of an object.
>
>> > Having str(cls) return just the class name (or the module.class
>> > dotted name) is an attractive nuisance that should be resisted.
>
> Agreed.
>
>> When I am printing an object and I have no idea what type it is, I'll
>> use repr() or "%r"; but when I know I am printing, say, an exception,
>> I think it would be very nice if print(x) would just print its name.
>
> -1. That makes the string representation of an exception much less
> useful.
>
> Exceptions don't have names; each exception *type* has a name, but that
> doesn't distinguish instances of the type from one another. When there
> is an 'IOError' it's far more useful that the string representation
> contains the exception *message*, since the name of the exception type
> doesn't tell the user much about what went wrong.

You misunderstood me. I'm not proposing to change the str() of an
exception *instance*. That will continue to be the message (while its
repr() is a <...> style thing). I'm only proposing to change the str()
of an exception *class* -- or any other class, for that matter.

>> Just like print(None) prints 'None', it would make all the sense in
>> the world if print(ZeroDivisionError) printed 'ZeroDivisionError'
>
> Those examples are objects which are effectively identical in each
> instance. (In the case of None, there is only one instance). Those are
> the unusual case; the common case is that objects of a given type are
> different form each other, and that frequently means their printable
> representation should be different too.

Right, and the built-in types and exceptions are in the same category.

> If the instances have a name (e.g. function objects), I agree the name
> should be *included in* the string representation. But there's usually
> other useful information that should also be included.

For that you can use repr().

-- 
--Guido van Rossum (python.org/~guido)