Recursion

It is legal for one function to call another; it is also legal for a function to call itself. It may not be obvious why what is a good thing, but it turns out to be one of the most magical things a program can do. For example:

If a recursion never reaches a base case, it goes on making recursive calls forever, and the program never terminates. This is known as infinite recursion, and it is generally not a good idea. In most programming environments, a program with infinite recursion does not really run forever. Python reports an error message when the maximum recursion depth is reached:

When the error occurs, there are 1000 recurse frames on the stack.

form Thinking in Python