1. Call Stacks, Arguments and Returns
And a bit about Recursion

2. Functions can call other functions!
It is a natural tendency to break a big problem up into smaller problems
This can be repeated several times – funA can be broken up into funA1 and funA2 and funA3, which can be broken into funA11 and funA12 and so on
The execution proceeds just as in other examples of function calls.
The interpreter keeps track of all these function calls using a data structure called a “call stack”.

3. More semantics - Call stacks (stack frames)
To keep track of all the activity involved in a function call, the interpreter uses a data structure called a “call stack”
It is a stack of records, each record is about a function call.
When a function call is made, a record of that fact is put on top of the call stack. That indicates that that function is the one executing.
When a function call is finished and is ready to return, its record is removed from the top of the stack to say it is no longer the one running. The record underneath that is the one belonging to the function which called.

4. More semantics – Arguments and Parameters
Parameters are named in the def statement (can only be identifiers)
They are placeholders, waiting to be filled in when the function runs
There can be any number of parameters (but don’t go overboard! more than 10 is excessive)
Arguments are named in the function call
Arguments have values which are copied to the parameters
Arguments can be identifiers, expressions, constants
The number of arguments must match the number of parameters
The names of arguments do NOT have to match the names of the parameters
The types of arguments and parameters should match or at least be similar

5. The return statement
The return statement tells Python that the execution of the current function (the function that the return statement is in) is done and that control passes back to the function which called it
The return statement can have nothing after the keyword, in which case Python returns the special value None
Or the return statement can have one or more expressions after the keyword (yes, a Python function can return multiple values!)
When the return statement is executed, the function is finished – therefore, do NOT put code after a return statement, it will never execute!

6. Recursion Functions can call themselves! That is called Recursion.
See chapter 12 for details.
Recursion is a powerful technique but must be written carefully or it will cause infinite loops.
Don’t worry about recursion until it is covered in class