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

[Michael Foord]

On 22 October 2011 21:32, Guido van Rossum <guido at python.org> wrote:

> On Sat, Oct 22, 2011 at 1:18 PM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> > Nick Coghlan wrote:
> >
> >> If someone wants debugging level detail, use repr(), just like the
> >> interactive interpreter does.
> >
> > 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().
> >
> > The details returned by str() are, in some sense, arbitrary. The docs
> > describe it as [quote] the “informal” string representation of an object
> > [end quote].
> >
> > http://docs.python.org/reference/datamodel.html#object.__str__
> >
> > On that basis, objects are free to return as much, or as little,
> information
> > as makes sense in their str(). (As you pointed out earlier.)
> >
> > However, the docs also say that str() should return [quote] a string
> > containing a nicely printable representation of an object [end quote].
> >
> > http://docs.python.org/library/functions.html#str
> >
> > To my mind, the name alone of a class (or function or module) is in no
> sense
> > a nicely printable representation of the object. I would argue strongly
> that
> > the property of being "nicely representable" outweighs by far the
> > convenience of avoiding 9 extra characters in one specific use-case:
> >
> > "blah blah blah class '%s'" % cls  # instead of cls.__name__
> >
> >
> > But for the sake of the argument, I'll grant you that we're free to
> change
> > str(cls) to return the class name, as requested by the OP, or the fully
> > qualified module.class dotted name as suggested by you. So let's suppose
> > that, after a long and bitter debate over which colour to paint this
> > bikeshed, you win the debate.
> >
> > But this doesn't help you at all, because you can't rely on it. It seems
> to
> > me that the exact format of str(cls) is an implementation detail. You
> can't
> > rely on other Pythons to do the same thing, nor can you expect a
> guarantee
> > that str(cls) won't change again in the future. So if you care about the
> > exact string that gets generated, you still have to explicitly use
> > cls.__name__ just as you do now.
> >
> > The __name__ attribute is part of the guaranteed API of class objects
> (and
> > also functions and modules), the output of str(cls) is not. In my opinion
> > relying on it to return a particular output is dangerous, regardless of
> > whether the output is "<class 'module.MyClass'>", "module.MyClass",
> > "MyClass" or something else.
> >
> > Having str(cls) return just the class name (or the module.class dotted
> name)
> > is an attractive nuisance that should be resisted.
>
> Thinking of str(x) as an API to get a certain value would lead there,
> yes. But thinking of str(x) as what gets printed by print(x),
> formatted by "{}.format(x)", and "%s" % s, changes things. 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. Just like
> print(None) prints 'None', it would make all the sense in the world if
> print(ZeroDivisionError) printed 'ZeroDivisionError', and
> print(type(42)) printed 'int'.
>
>
+1

Michael



> --
> --Guido van Rossum (python.org/~guido <http://python.org/%7Eguido>)
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