python format_spec syntax [[fill]align]

Question

What does this ( [[fill]align]) syntax mean?

From the format specification mini language https://docs.python.org/2/library/string.html#format-specification-mini-language

Does it mean if you have a fill then you must have an align?

I was trying -

In [71]: '{:{}{}}'.format('test','.','10')
Out[72]: 'test'

I think that has a fill(character) '.' and width ('10'). I used python 3.6.3.

Answer 1

I think perhaps you want to look the the examples here .

The fill characters and the justification parameters go in as part of the string. For example

In [56]: '{:.<10}'.format('test')
Out[56]: 'test......'

In [57]: '{:.>10}'.format('test')
Out[57]: '......test'

If you want to dynamically generate the format string you could do something like this,

In [76]: '{:{}<{}}'.format('test','.','10')
Out[76]: 'test......'

Answer 2

Yes, if you have a fill, you must have an align. The . in your test is not treated as a fill, because you are missing an align; instead, the .10 is parsed as the 10 specifying a precision value. See the [.precision] in the format spec syntax :

format_spec ::=  [[fill]align][sign][#][0][width][,][.precision][type]
                                                    ^^^^^^^^^^^^

For a string, a precision value of 10 will truncate the string to 10 characters:

>>> '{:{}{}}'.format('123456789012345','.','10')
'1234567890'

If you had an align, then the . would be treated as a fill:

>>> '{:{}>{}}'.format('test','.','10')
'......test'