i can not understand how replace function work ??

Answer 1

imagine you have:

s = "I like Python."

You want to replace the word 'like' to 'hate'. You do:

s.replace('like', 'hate')
print(s) # output: I hate Python

Now imagine:

s = "programming is challenging and challenging is good."

Now you want to replace any occurence of 'is' for 'was'. You do:

s.replace('is', 'was', 2) # the third argument is the number of ocurrences to replace.

Hope this helps.

Answer 2

You can leave off the optional third parm to replace all occurrences