[Python-Dev] bpo-34595: How to format a type name? (original) (raw)

Petr Viktorin encukou at gmail.com
Thu Sep 13 17:52:42 EDT 2018

On 09/13/18 14:08, Victor Stinner wrote:

Le jeu. 13 sept. 2018 à 16:01, Eric V. Smith <eric at trueblade.com> a écrit :

* Add !t conversion to format string

I'm strongly opposed to this. This !t conversion would not be widely applicable enough to be generally useful, and would need to be exposed in the f-string and str.format() documentation, even though 99% of programmers would never need or see it. (I'm thinking aloud.) In the Python code base, I found 115 lines using type(obj).name and 228 lines using obj.class.name. [...]

"!t" is not a big improvement over ":T" and "type(obj)".

I'm not sure if type(obj) or obj.class should be used, but I can say that they are different: obj.class can be overriden: [...]

Moreover, it's also possible to override the "type" symbol in the global or local scope: [...]

I don't think either of those are problematic. If you override __class__ or type, things will behave weirdly, and that's OK.

One advantage of having a builtin formatter would be to always use internally the builtin type() function to get the type of an object, or not use "type()" in the current scope. The second advantage is to prevent the need of having to decide between type(obj) and obj.class :-)

raise TypeError(f"must be str, not {obj!t}") Should be written as: raise TypeError(f"must be str, not {type(obj)}") [...] Do you want to modify str(type) to return a value different than repr(type)? Or maybe it's just a typo and you wanted to write f"{type(obj):T}"?

Yes, AFAIK that was a typo.

I think "T" is a good idea, but I think you're adding in obj vs type(obj) just because of the borrowed reference issue in PyTYPE(). That issue is so much larger than string formatting the type of an object that it shouldn't be addressed here. Right, that's a side effect of the discussion on the C API. It seems like PyTYPE() has to go in the new C API. Sorry, the rationale is not written down yet, but Dino convinced me that PyTYPE() has to go :-)

I'll be happy when we get rid of Py_TYPE and get to use moving garbage collectors... but now is not the time. The API for "%T" should be "give me the type". The best way to do that might change in the future.

But at this point, we're bikeshedding. I think all the relevant voices have been heard.

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