"Can't assign to function call"

Question

My function is supposed to take a string argument as input, and return the rot-13 encoding of the input string.

def str_rot_13(string):

    c = list(string)

    for num in c:
       if ord(num) >= ord('a') and ord('z'):
          if ord(num) >=('m'):
             ord(num) -=13
          else:
             ord(num) +=13
    
       elif ord(num) >= ord('A') and ord('Z'):
          if ord(num) >=('M'):
             ord(num) -=13
          else:
             ord(num) +=13

    z += chr(ord(num))
    return z

It's giving me a "can't assign to function call" error. I'm not sure what I'm doing wrong.

Answer 1

The problem is with lines like this one:

ord(num) -=13

ord is a built-in function. You can use a value returned by a function, but not assign a value to a function.

What you can do instead is:

num = chr(ord(num) - 13)

This will probably not solve your problem, as you have other bugs, like you are trying to add to variable z without declaring it somewhere. You should declare it before your for loop:

z = ''
for num in c:
...

and also indent the line

z += chr(ord(num))

so that it is inside the for loop. You can also make it:

z += num

as chr and ord are reverse functions.

Answer 2

What you're doing wrong is, you're assigning to a function call! E.g:

ord(num) -=13

you're assigning to the function call ord(num) -- and, you can't do it.

What you actually want to do is presumably:

num = chr(ord(num) - 13)

and so on.

Of course, you'll still have problems appending to z unless you define z in a part of the code you chose not to show us. Hard to help debug code you choose to hide from us, of course.