To ensure that a function is defined before its first use, you have to know the order statements run in, which is called the flow of execution.
Execution always begins at the first statement of the program. Statements are run one at a time, in order from top to bottom.
Function definitions do not alter the flow of execution of the program, but remember that statements inside the function don’t run until the function is called.
A function call is like a detour in the flow of execution. Instead of going to the next statement, the flow jumps to the body of the function, runs the statements there, and then comes back to pick up where it left off.
That sounds simple enough, until you remember that one function can call another. While in the middle of one function, the program might have to run the statements in another function. Then, while running that new function, the program might have to run yet another function!
Fortunately, Python is good at keeping track of where it is, so each time a function completes, the program picks up where it left off in the function that called it. When it gets to the end of the program, it terminates.
In summary, when you read a program, you don’t always want to read from top to bottom. Sometimes it makes more sense if you follow the flow of execution.