To substitute a substring old by a substring new we can use the method replace:
s.replace(old, new[, count])
Without using the optional parameter count every occurence of old will be replaced by new.
>>> s = "jkh abc jklj abc jkkj klj abc jlj abc iii"
>>> s.replace("abc", "---")
'jkh --- jklj --- jkkj klj --- jlj --- iii'
The optional parameter count can be used to control how many occurences of old will be replaced by new.
>>> s.replace("abc", "---", 3)
'jkh --- jklj --- jkkj klj --- jlj abc iii'