Python String replace

I came across a point where the incoming user string had to be replaced with something i desire . Considering the simplicity of the activity i expected python to have some user defined function . might be a simple thing , but really useful...

replace( old , new, [max count]).
if you have the below
Code :
sample = "this is a test string"
print sample.replace("test" , "long test")
the result will look like this
"this is a long test string"
you dont have to import any library in specific.
the "max" from the syntax constitutes to the number of times the string has to be replaced.