How can I fill out a Python string with spaces?

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        '

For a flexible method that works even when formatting complicated string, you probably should use the string-formatting mini-language, using either the str.format() method

>>> '{0: <16} StackOverflow!'.format('Hi')  # Python >=2.6
'Hi               StackOverflow!'

of f-strings

>>> f'{"Hi": <16} StackOverflow!'  # Python >= 3.6
'Hi               StackOverflow!'