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Tue May 6 20:40:40 CEST 2014 · https://mail.python.org/pipermail/python-dev/2014-May/134445.html

On 05/06/2014 10:33 AM, James Swift wrote:
> Hi,
> 
> In 3.3 I could do the following
> 
>>>> "{x:s}".format(**{'x': [1, 2, 3]})
> '[1, 2, 3]'
> 
> But in 3.4
> 
>>>> "{x:s}".format(**{'x': [1, 2, 3]})
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: non-empty format string passed to object.__format__
> 
> 
> Is this intentional?

Yes. See http://bugs.python.org/issue7994 for the rationale.

You can use:
>>> "{x!s:15s}".format(**{'x': [1, 2, 3]})
'[1, 2, 3]      '

That is, convert it first to a string (with !s) then use whatever string
formatting specifier you want (here, '15s'). This should work under all
3.x versions (I think, haven't checked).

BTW, for this specific case, you might want to use format_map:
>>> "{x!s:15s}".format_map({'x': [1, 2, 3]})
'[1, 2, 3]      '

Eric.

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[Python-Dev] Behaviour change of object().format()