Python: string replace

str = "this is string example....wow!!! this is really string";


print str.replace("is", "was");    
# thwas was string example....wow!!! thwas was really string


print str.replace("is", "was", 3); 
# thwas was string example....wow!!! thwas is really string


Source:
http://www.tutorialspoint.com/python/string_replace.htm