How can the built-in range function take a single argument or three?

How can the range function take either: a single argument, range(stop), or range(start, stop), or range(start, stop, step). Does it use a variadic argument like *arg to gather the arguments, and then use a series of if statements to assign the correct values depending on the number of arguments supplied? In essence, does range() specify that if there is one argument, then it set as the stop argument, or if there are two then they are start, and stop, or if there are three then it sets those as stop, start, and step respectively? I'd like to know how one would do this if one were to write range in pure CPython.