what is the output of this code and how ?

Question

lst=["1","2","3"]
lst[ : ]="999"
print(lst)

Answer 1

This is basically the same as:
lst = list("999")

It takes an iterable and appends each of its value to the list, in this case "999" is iterable so the list will become:
['9', '9', '9']

Answer 2

It print ["9","9","9"]
It is because lst takes value one by one from string and make list . Here lst[ : ] means starting index value 0 to end .