[Python-Dev] PEP 498 f-string: please remove the special case for spaces

[Eric V. Smith]

On 8/10/2015 8:00 PM, MRAB wrote:
> On 2015-08-11 00:26, Victor Stinner wrote:
>> Le mardi 11 août 2015, Eric V. Smith <eric at trueblade.com
>> <mailto:eric at trueblade.com>> a écrit :
>>
>>     It sounds like you want to disallow leading spaces just to
>>     disallow this one type of expression.
>>
>>
>>   I would like to reduce the number of subtle differences between
>> f-string and str.format().
>>
> I'm a little bit surprised at seeing this:
> 
>>>> '{0}'.format('foo')
> 'foo'
>>>> '{ 0}'.format('foo')
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> KeyError: ' 0'
>>>> '{a}'.format(a='foo')
> 'foo'
>>>> '{ a}'.format(a='foo')
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> KeyError: ' a'
> 
> In some other cases, leading and trailing spaces are ignored:
> 
>>>> int(' 0 ')
> 0
> 
> Outside string literals, they're also ignored.
> 
> But, then:
> 
>>>> '{-1}'.format('foo')
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> KeyError: '-1'
> 
> It's a string key, even though it looks like an int position.

I think there are bug tracker issues for both of these. I think the
argument against changing them is that people might be depending on this
behavior. I'll grant you it seems unlikely, but you never know.

Eric.