Cheat sheet for the new string formatting?

[Serhiy Storchaka]

On 08.06.15 23:32, Skip Montanaro wrote:
> This is counterintuitive:
>
>  >>> "{:.3}".format(-0.00666762259822)
> '-0.00667'
>  >>> "{:.3f}".format(-0.00666762259822)
> '-0.007'
>  >>> "%.3f" % -0.00666762259822
> '-0.007'
>  >>> "{:.3s}".format(-0.00666762259822)
> ValueError Unknown format code 's' for object of type 'float'
>
> Why does the first form display five digits after the decimal point? Why
> don't floats support "{:.Ns}"? (I know I can use "{!s}".)

"{:.3}" for floats is equivalent to "{:.3g}".

 >>> "{:.3g}".format(-0.00666762259822)
'-0.00667'
 >>> "%.3g" % -0.00666762259822
'-0.00667'

Format code 's' would produce incorrect and meaningless output for floats.

 >>> "{!s:.3}".format(-0.00666762259822)
'-0.'
 >>> "%.3s" % -0.00666762259822
'-0.'
 >>> "{!s:.3}".format(-0.0000666762259822)
'-6.'
 >>> "%.3s" % -0.0000666762259822
'-6.'