input: Write a function to calculate the square root of a user-defined positive number. If the number is not positive, return an appropriate message. If the number is empty, return an appropriate message.
  assert square_root(100)==10.0
  assert square_root(25)==5.0
  assert square_root(-10)=="Please enter a positive number"
  assert square_root(0)=="Please enter a positive number"
  assert square_root([])=="Please enter a positive number"
output: def square_root(num):
      if not num or num <= 0:
          return "Please enter a positive number"
      return num ** 0.5