[Python-Dev] bpo-34595: How to format a type name?

Eric V. Smith eric at trueblade.com
Thu Sep 13 20:24:00 EDT 2018

On 9/13/2018 8:04 PM, Victor Stinner wrote:

> Le ven. 14 sept. 2018 à 00:09, Eric V. Smith <eric at trueblade.com> a écrit :
>> f'{type(obj)}' becomes type(obj).__format__(''), so you can return
>> something other than __str__ or __repr__ does. It's only by convention
>> that an object's __format__ returns __str__: it need not do so.
> What's New in Python 3.7 contains:
>> object.__format__(x, '') is now equivalent to str(x) rather than format(str(self), '').
>> (Contributed by Serhiy Storchaka in bpo-28974.)
> https://bugs.python.org/issue28974
> Oh, I didn't know that a type is free to change this behavior: return
> something different than str(obj) if the format spec is an empty
> string.

True! That issue was specific to object.__format__, not any other 
classes implementation of __format__.

> So are you suggesting to change type(obj).__format__('') to return the
> fully qualified name instead of repr(type)?

I'm not suggesting it, I'm saying it's possible. It indeed might be the 
most useful behavior.

> So "%s" % type(obj) would use repr(), but "{}".format(type(obj)) and
> f"{type(obj)}" would return the fully qualified name?

"%s" % type(obj) would use str(), not repr.

You could either:
- keep with convention and have type(obj).__format__('') return type(obj).__str__(), while type(obj).__format__('#') (or what other char you want to use) return the qualname;
- just have type(obj).__format__('') return the qualname, if that's the more useful behavior.


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