old_contents: #!/usr/bin/env python3
  # -*- coding: utf-8 -*-
  
  
  def calculator(op, a, b):
      if op == '+':
          return a + b
      elif op == '-':
          return a - b
      elif op == '*':
          return a * b
      elif op == '/':
          return a / b
  
new_contents: #!/usr/bin/env python3
  # -*- coding: utf-8 -*-
  
  
  def calculator(op, a, b):
      if op == '+':
          return a + b
      elif op == '-':
          return a - b
      elif op == '*':
          return a * b
      elif op == '/':
          return a / b
  
  
  def some_func():
      '''This function is not tested'''
      do = 'something'
      here = ...
      for i in range(10):
          b = i + 1
          print(b)
  
subject: Add non-tested function