variable number of digit in format string

Question

Which is a clean way to write this formatting function:

def percent(value,digits=0):
    return (\'{0:.%d%%}\' % digits).format(value)

>>> percent(         


        

Answer 1

I like this one:

'{0:.{1}%}'.format(value, digits)

Test:

>> '{0:.{1}%}'.format(0.1565, 0)
'16%'
>> '{0:.{1}%}'.format(0.1565, 2)
'15.65%'

Answer 2

From the docs:

Minimum field width (optional). If specified as an '*' (asterisk), the actual width is read from the next element of the tuple in values, and the object to convert comes after the minimum field width and optional precision.

Example:

def percent(value, digits=0):
    print '%.*f%%' % (digits, value*100)
>>> percent(0.1565, 2)
15.65%

Answer 3

* does what you want, for printf-style string formatting.

>>> def percent(value, digits=0):
...     return '%.*f%%' % (digits, value * 100)
...
>>> percent(0.1565, 2)
'15.65%'

Advanced string formatting (defined in PEP 3101 and documented in 7.1.3. Format String Syntax) doesn't seem to be capable of doing this in one pass. (See 7.1.3.1. Format Specification Mini-Language: precision is integer only.)