The formal function arguments specified when a function is defined with the def keyword are a comma-separated list of the argument names. These arguments can be made optional by providing default values. Consider a function which prints a simple banner to the console:

>>> def banner(message, border='-'):
...     line = border * len(message)
...     print(line)
...     print(message)
...     print(line)
...

This function takes two arguments, and we provide a default value — in this case '-' — in a literal string. When we define functions using default arguments, the parameters with default arguments must come after those without defaults, otherwise we will get a SyntaxError.

On line 2 of the function we multiply our border string by the length of the message string. This line shows two interesting...