Problem with my Python code: IndexError: string index out of range

Question

Recently I was working on a python challenge where I needed to create a function that was capable of transcribing DNA to mRNA, but every time I run the code I receive the same error message:

if dna[y] == "A": IndexError: string index out of range

Can anyone help me? Below is a sample of my code.

global y
y = 0
global rna
rna = ""
def dna_to_rna(dna):
    global y
    global rna
    for i in range(len(dna)):
        if dna[y] == "A":
            rna += "U"
        elif dna[y] == "T":
            rna += "A"
        elif dna[y] == "G":
            rna += "C"
        else:
            rna += "G"
        y += 1
    return rna  

Answer 1

You have a lot of extra variables and you don't need 'global' modifiers in your code. I've done some reformatting to your code, try this:

def dna_to_rna(dna: str) -> str:
  rna = ""
  for i in range(len(dna)):
      if dna[i] == "A":
          rna += "U"
      elif dna[i] == "T":
          rna += "A"
      elif dna[i] == "G":
          rna += "C"
      elif dna[i] == "C":
          rna += "G"
      else:
          return "Invalid Sequence!"
  return rna