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[Python-Dev] Default formatting

Serhiy Storchaka storchaka at gmail.com
Sun Jan 22 14:48:15 EST 2017 
On 25.10.16 12:37, Serhiy Storchaka wrote:
> Classes that doesn't define the __format__ method for custom PEP 3101
> formatting inherits it from parents.
>
> Originally the object.__format__ method was designed as [1]:
>
>     def __format__(self, format_spec):
>         return format(str(self), format_spec)
>
> An instance is converted to string and resulting string is formatted
> according to format specifier.
>
> Later this design was reconsidered [2], and now object.__format__ is
> equivalent to:
>
>     def __format__(self, format_spec):
>         assert format_spec == ''
>         return format(str(self), '')
>
> Non-empty format specifier is rejected.
>
> But why call format() on resulting string? Why not return resulting
> string as is? object.__format__ could be simpler (not just
> implementation, but for understanding):
>
>     def __format__(self, format_spec):
>         assert format_spec == ''
>         return str(self)
>
> This can change the behaviour in corner case. str(self) can return not
> exact string, but string subclass with overloaded __format__. But I
> think we can ignore such subtle difference.
>
> [1] https://www.python.org/dev/peps/pep-3101/
> [2] http://bugs.python.org/issue7994

What is the decision about this?
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