Chris Wilcox <pyt...@crwilcox.com> added the comment:
The attached code implements `__format__` on the `Collections` class. In case
1, the template passed to `__format__` is "{v.name}: {v.email}|". In case 2, a
name error will occur while processing the f string and v will not be found as
no object 'v' exists in locals or globals.
In reviewing PEP 0498, https://www.python.org/dev/peps/pep-0498/, I think the
difference is in what object is being formatted.
In case 1 of the attached code, the collection is being formatted. In case 2
where f-strings are used, 'v' is being formatted. Because v doesn't exist in
this context, it fails. I found this in the PEP and I think it is what is going
on here.
```
Note that __format__() is not called directly on each value. The actual code
uses the equivalent of type(value).__format__(value, format_spec), or
format(value, format_spec). See the documentation of the builtin format()
function for more details.
```
----------
nosy: +crwilcox
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<https://bugs.python.org/issue39601>
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