[Python-3000] PEP 3101 Updated

[Ron Adam]

Eric Smith wrote:
> Ron Adam wrote:
>>> I've been re-reading the PEP, in an effort to make sure everything is 
>>> working.  I realized that these tests should not pass.  The PEP says 
>>> that "Format specifiers can themselves contain replacement fields".  
>>> The tests above have replacement fields in the field name, which is 
>>> not allowed.  I'm going to remove this functionality.
>>>
>>> I believe the intent is to support a replacement for:
>>> "%.*s" % (4, 'how now brown cow')
>>>
>>> Which would be:
>>> "{0:.{1}}".format('how now brown cow', 4)
>>>
>>> For this, there's no need for replacement on field name.  I've taken 
>>> it out of the code, and made these tests in to errors.
>>
>> I think it should work myself, but it could be added back in later if 
>> there is a need to.
>>
>>
>> I'm still concerned about the choice of {{ and }} as escaped brackets.
>>
>> What does the following do?
>>
>>
>> "{0:{{^{1}}".format('Python', '12')
> 
>  >>> "{0:{{^{1}}".format('Python', '12')
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> ValueError: unterminated replacement field

When are the "{{" and "}}" escape characters replaced with  '{' and '}'?

> But:
>  >>> "{0:^{1}}".format('Python', '12')
> '   Python   '
 >

>> "{{{0:{{^{1}}}}".format('Python', '12')
>  >>> "{{{0:{{^{1}}}}".format('Python', '12')
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> ValueError: Unknown conversion type }
> 
> But,
>  >>> "{{{0:^{1}}".format('Python', '12')
> '{   Python   '

So escaping '{' with '{{' and '}' with '}}' doesn't work inside of format 
expressions?

That would mean there is no way to pass a brace to a __format__ method.


>> class ShowSpec(str):
>>        
>>     return spec
>>
>> ShowSpec("{0:{{{1}}}}").format('abc', 'xyz')
>>
> 
>  >>> ShowSpec("{0:{{{1}}}}").format('abc', 'xyz')
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> ValueError: Invalid conversion specification

> I think you mean:
> ShowSpec("{0:{1}}").format('abc', 'xyz')

No, because you may need to be able to pass the '{' and '}' character to 
the format specifier in some way.  The standard specifiers don't use them, 
but custom specifiers may need them.


> But I have some error with that.  I'm looking into it.
> 
>> "{0}".format('{value:{{^{width}}', width='10', value='Python')
> 
>  >>> "{0}".format('{value:{{^{width}}', width='10', value='Python')
> '{value:{{^{width}}'

Depending on weather or not the evaluation is recursive this may or may not 
be correct.

I think it's actually easier to do it recursively and not put limits on 
where format specifiers can be used or not.

_RON