How to properly use string indices to create a rot13 equivalent of a string

Question

For my programming class, we have to create a function that takes in a string argument and returns the rot13 equivalent of the string. When I try running my function, it says that count can't equal str[i] because string indices have to be integers. I'm honestly lost and what else I could do to make the function work. Any help would be lovely

def str_rot_13(str):
     new_list = []
     for i in str:
         if ord(i) <= 77:
             count = str[i]
             k = chr(ord(count) + 13)
             new_list.append(k)
         if ord(i) > 77 and ord(i) <= 90:
             count = str[i]
             k = ord(count) - 78
             new_list.append(chr(65 + k))
     return new_list

Answer 1

 for i in str:
     if ord(i) <= 77:
         count = str[i]
         k = chr(ord(count) + 13)

In Python, for i in str will loop through every character in the string str, with i set to that character (which you already know, since you're doing ord(i)). (Don't use str as a name, by the way: str is the Python name for the string type.) count = str[i] is treating the character in i as an index. You don't need to (or should) do that. It doesn't make much sense.