We have seen built-in functions that take a variable number of arguments. For example range can take one, two or three arguments.

It is possible to write user-defined functions with optional arguments, too. For example here is a function that prints the most common words in a histogram:

If you provide one argument number gets the default value. If you provide two arguments, num gets the value of the argument instead. In other words, the optional argument overrides the default value.

If a function has both required and optional parameters, all the required parameters have to come first, followed by the optional ones.

from Thinking in Python