Python - use format string length variable instead of fixed one?

Question

For example I can do this:

"%.2s" % ('aaa')
'aa'

or

"%.1s" % ('aaa')
'a'

But how can I make that part like .2 variable, so I could pass any number and it would format accordingly, like if I would pass something like:

"%.%ss %" (1, 'aaa') # Pseudo code
'a'

I woul take a look over String.format method : docs.python.org/2/library/… — Netwave
– Netwave, Commented Nov 5, 2015 at 8:17

Martijn Pieters · Accepted Answer · 2015-11-05 08:20:32Z

4

Specify the width as * and the actual width will be taken from the next positional argument:

"%.*s" % (1, 'aaa')

The str.format() method is more flexible still, you can interpolate any parameter, not just the width:

"{:.{width}s}".format('aaa', width=1)

Demo:

>>> "%.*s" % (1, 'aaa')
'a'
>>> "%.*s" % (2, 'aaa')
'aa'
>>> "{:.{width}s}".format('aaa', width=1)
'a'
>>> "{:.{width}s}".format('aaa', width=2)
'aa'

The extra placeholders can be used for any element making up the format specification:

>>> "{:{align}{width}s}".format('aaa', align='<', width=4)
'aaa '
>>> "{:{align}{width}s}".format('aaa', align='>', width=4)
' aaa'

answered Nov 5, 2015 at 8:20

Martijn Pieters

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1 Comment

Nice. Thanks. Both solutions are good, though first one seems like is more readable in this case (at least for me).