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

Eric V. Smith eric at trueblade.com
Thu Dec 19 20:06:53 CET 2013

On 12/16/2013 03:49 PM, Nick Coghlan wrote:
> On 17 Dec 2013 02:23, "Eric V. Smith" <eric at trueblade.com
> <mailto: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.

That's an excellent observation, Nick, including that it should be based
on a builtin. But I'd suggest something like classof(), and have it's
__format__ "do the right thing". But it all seems like overkill for this

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

True enough!


> Cheers,
> Nick.
>> 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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