[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
Mon Dec 16 21:49:38 CET 2013

On 17 Dec 2013 02:23, "Eric V. Smith" <eric at trueblade.com> wrote:
> On 12/16/2013 10:29 AM, Walter Dörwald wrote:
> > I'd vote for including the module name in the string and using
> > __qualname__ instead of __name__, i.e. make "{:T}".format(obj)
> > equivalent to
> > "{0.__class__.__module__}.{0.__class__.qualname__}".format(obj).
> That's not possible in general. The format specifier interpretation is
> done by each type. So, you could add this to str.__format__ and
> int.__format__, but you can't add it to an arbitrary type's __format__.
> For example, types not in the stdlib would never know about it.

That just suggests it would need to be a type coercion code, like !a, !r,
and !s. However, that observation also suggests that starting with a
"classname" or "typename" builtin would be more appropriate than jumping
directly to a formatting code.

We've definitely drifted well into python-ideas territory at this point,
though :)


> There's no logic for calling through to object.__format__ for unknown
> specifiers. Look at datetime, for example. It uses strftime, so "T"
> currently just prints a literal "T".
> And for object.__format__, we recently made it an error to specify any
> format string. This is to prevent you from calling
> format(an_object, ".30")
> and "knowning" that it's interpreted by str.__format__ (because that's
> the default conversion for object.__format__). If in the future
> an_object's class added its own __format__, this code would break (or at
> least do the wrong thing).
> But I really do like the idea! Maybe there's a way to just make
> obj.__class__ recognize "T", so you could at least do:
> format(obj.__class__, "T")
> or equivalently:
> "{:T}".format(obj.__class__)
> I realize that having to use .__class__ defeats some of the beauty of
> this scheme.
> Eric.
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