> Yes, that makes some sense.  How would you handle 
> '{:.10}'.format(10/3.), though?  We could either change 's' to allow a 
> precision, or use 's' when there's no precision specified and 'g' (with 
> the ADD_DOT_0 flag) otherwise.

Good point, I hadn't thought of that. I'm not a big fan of switching 
between 's' and 'g' depending on whether a precision is specified.

>> Or, also switch float_str to use 'g' with a precision of
>> PyFloat_STR_PRECISION, and get rid of 's' altogether.
> 
> This sounds good to me.  It does feel as though there's unnecessary
> duplication with the current setup.

A major point of 's' was to not specify the precision, so I'd prefer to 
remove 's' and use 'g' with a specified precision.