Function Design Recipe

The Six Steps

  1. Examples
    • What should your function do?
    • Type a couple of example calls.
    • Pick a name (often a verb or verb phrase): What is a short answer to "What does your function do"?
  2. Type Contract
    • What are the parameter types?
    • What type of value is returned?
  3. Header
    • Pick meaningful parameter names.
  4. Description
    • Mention every parameter in your description.
    • Describe the return value.
  5. Body
    • Write the body of your function.
  6. Test
    • Run the examples.

Applying the Design Recipe

The problem:

The United States measures temperature in Fahrenheit and Canada measures it in Celsius. When travelling between the two countries it helps to have a conversion function. Write a function that converts from Fahrenheit to Celsius.

  1. Examples 
        >>> convert_to_celsius(32)
    0
    >>> convert_to_celsius(212)
    100
  2. Type Contract 
        (number) -> number
  3. Header 
        def convert_to_celsius(fahrenheit):
  4. Description 
        Return the number of Celsius degrees equivalent to fahrenheit degrees.
  5. Body 
            return (fahrenheit - 32) * 5 / 9
  6. Test 
         Run the examples.

Putting it all together:

def convert_to_celsius(fahrenheit):
   ''' (number) -> number
   
   Return the number of Celsius degrees equivalent to fahrenheit degrees.
   
   >>> convert_to_ccelsius(32)
   0
   >>> convert_to_celsius(212)
   100
   '''
   
   return (fahrenheit - 32) * 5 / 9