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Thu Oct 27 12:59:39 EDT 2016 · https://mail.python.org/pipermail/python-dev/2016-October/146769.html

On 27Oct2016 0251, Serhiy Storchaka wrote:
> On 27.10.16 02:44, Eric V. Smith wrote:
>> But on the other hand, the existing behavior is well specified and has
>> been around since object.__format__ was added. I'm not sure it needs
>> changing. What's the harm in leaving it?
>
> More complicated code. And maybe this behavior is less intuitive. It
> contradicts the documentation.
>
> From the documentation of the format() builtin [1]:
>
> "The default format_spec is an empty string which usually gives the same
> effect as calling str(value)."
>
> From the description of the format specification mini-language [2]:
>
> "A general convention is that an empty format string ("") produces the
> same result as if you had called str() on the value."
>
> [1] https://docs.python.org/3/library/functions.html#format
> [2] https://docs.python.org/3/library/stdtypes.html#str.format
>

The only point where this bothers me is that alignments don't work:

 >>>  class F: pass
...
 >>>  '{}'.format(F())
'<__main__.F object at 0x000002148AFE6B70>'
 >>>  '{:100}'.format(F())
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: non-empty format string passed to object.__format__
 >>>  '{:<100}'.format(F())
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: non-empty format string passed to object.__format__

You need to explicitly include '!s' to be able to align it, which then 
means overriding __format__ in that class later won't have any effect.

 >>>  '{!s:<100}'.format(F())
'<__main__.F object at 0x000002148AFEE240> 
                             '

Having the default __format__ behave like this makes me happiest:

...     def __format__(self, fmt):
...         return format(str(self), fmt)

My 2c. YMMV. etc.

Cheers,
Steve

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[Python-Dev] Default formatting