bug in ''.format()

[MRAB]

On 2017-11-02 21:30, Ken Kundert wrote:
> I just encountered this:
> 
>>>> '{:0s}'.format('hello')
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> ValueError: '=' alignment not allowed in string format specifier
> 
> The exception goes away if I do not specify a width of the string:
> 
>>>> '{:s}'.format('hello')
> 'hello'
> 
> My reading of the documentation says I should be able to specify a width
> when interpolating a string.
> 
> I have tried this in 2.7.13, 3.6.1 and 3.6.2 and it fails in all of them.
> 
> Is this a bug or am I confused?
> -Ken
> 
The docs say this:

"""
When no explicit alignment is given, preceding the width field by a zero 
('0') character enables sign-aware zero-padding for numeric types. This 
is equivalent to a fill character of '0' with an alignment type of '='.
"""

It thinks that the "0" means zero-padding, etc, as explained above. 
(Having an explicit width of 0 is kind of odd anyway, if you think about 
it!)

The solution is to include an alignment character:

 >>> '{:<0s}'.format('hello')
'hello'