When you are struggling to define a function, it helps to have a plan. You might want to print this page out and keep it by your side.

Write the parts of the function in this order:

1. Function Header: Have you written the header, including…
   • def Keyword
   • Function name
   • Open parentheses: (
   • Parameters: Remember, each parameter needs to have:
     1. Parameter name
     2. Colon: :
     3. Parameter type
     4. Comma: But only between parameters.
   • End parentheses: )
   • Arrow: ->
   • Return type
   • Colon: :
2. Write a Docstring: Put a triple-quoted string literal (""" Like this """) inside of the function, indented with four spaces. Inside the literal, write a description of what the function does. Then unindent the next line to leave the function.
3. Write a Unit Test: Have you written at least one test case, including…
   • Calling assert_equal function
   • Calling your function: Make sure you call the function you are testing as the first argument to assert_equal.
   • Arguments: Your test case’s inputs are the arguments to your function.
   • Expected Value: The second argument to assert_equal should be a value that you expect your function to return.
4. Make More Unit Tests: Make several more test cases, trying to think about:
   • A simple case that gives an example of the function under normal circumstances
   • A case where integers are zero, strings are the empty string, or lists are empty
   • A more complex case using unusual numbers, symbols, or other values
5. Body: Now, you can think about what to put in the body, making sure you end with…
   • Return Statement: If your function does not use the return statement with a value, then it will return None.