Question

When using string formatting resulting string is longer than expected.

For example:

In [1]: "%s".ljust(7) % "123456"
Out[1]: '123456     '

Output string is not 7 characters long.

I’m interested in what is exactly happening ‘in the background’ to result in such behavior?

Answer 1

"%s" is being left adjusted to 7 places first – eg "%s ", and then you’re substituting the '123456' for the %s… hence the result you get…

>>> '%s'.ljust(7)
'%s     '
>>> _ % '123456'
'123456     '

You could use:

>>> ('%s' % '123456').ljust(7)
'123456

Or specify appropriate width in the format string…