What is the syntax to fill a string with a character, up until a certain length?

<string name>.center(<width>, <fillchar>)
#width is length of new string. If width longer than current length, then will use fillchar to fill in
#value to be added to existing strength 
str = 'hello world'
print(str.center(11, '-'))
print(str.center(13, '-'))
print(str.center(20, '@'))

hello world
-hello world-
@@@@hello world@@@@@