from math import sqrt

# Read a number, protecting the user from non-numeric input
# Parameters:
#   prompt: The prompt to display before accepting input
# Returns: A number entered by the user
def readNumber(prompt):
  # Use the success variable to mark whether or not a number has been read
  # successfully
  success = False

  # Keep looping until a number is entered
  while success == False:
    try:
      a = float(input(prompt))
      # If non-numeric input was entered then control jumps to the exception
      # handler.  The success varuable is only set to True when a number
      # is entered
      success = True

    except ValueError:
      print("That input wasn't numeric.  Enter the value again...")

  # Return the number entered by the user
  return a

# Read two numbers from the user in a way that protects against non-numeric
# input
a = readNumber("Enter a number: ")
b = readNumber("Enter another number: ")

# Perform several basic arithmetic operations
try:
  print(a,"+",b,"=",a+b)
  print(a,"-",b,"=",a-b)
  print(a,"*",b,"=",a*b)
  print(a,"/",b,"=",a/float(b))

except:
  # Display a nice error message if we attempt to divide by 0
  print("You can't divide by 0.")

try:
  print("sqrt(", a, ") = ", sqrt(a))

except ValueError:
  # Display a nice error message if we attempt to compute the square root of
  # a negative number
  print("You can't compute the square root of a negative number.")

print("The rest of the program.")