How can I fill out a Python string with spaces?

I want to fill out a string with spaces. I know that the following works for zero’s:

>>> print  "'%06d'"%4
'000004'

But what should I do when I want this?:

'hi    '

of course I can measure string length and do str+" "*leftover, but I’d like the shortest way.

You can do this with str.ljust(width[, fillchar]):

Return the string left justified in a string of length width. Padding is done using the specified fillchar (default is a space). The original string is returned if width is less than len(s).

>>> 'hi'.ljust(10)
'hi        '