str.replace(old, new[, max])

str = "this is string example....wow!!! this is really string"
print (str.replace("is", "was"))
print (str.replace("is", "was", 3))

thwas was string example....wow!!! thwas was really string
thwas was string example....wow!!! thwas is really string

import re
sourceline  = re.compile("Tutor", re.IGNORECASE)

Replacedline  = sourceline.sub("Tutor","Tutorialspoint has the best tutorials for learning.")
print (Replacedline)

Tutorialspoint has the best Tutorials for learning.