The way to invoke a function is to refer to it by name, followed by parentheses. Since there are no parameters for the function hello, we won’t need to put anything inside the parentheses when we call it. Once we’ve defined a function, we can call it as often as we like and its statements will be executed each time we call it.
First, note that in Step 1, when it executes line 1, it does not execute lines 2 and 3. Instead, as you can see in blue “Global variables” area, it creates a variable named hello whose value is a python function object. In the diagram that object is labeled hello() with a notation above it that it is a function.
At Step 2, the next line of code to execute is line 5. Just to emphasize that hello is a variable like any other, and that functions are python objects like any other, just of a particular type, line 5 prints out the type of the object referred to by the variable hello. It’s type is officially ’function’.
At Step 4 we get to line 8, which has an invocation of the function. The way function invocation works is that the code block inside the function definition is executed in the usual way, but at the end, execution jumps to the point after the function invocation.
You can see that by following the next few steps. At Step 5, the red arrow has moved to line 2, which will execute next. We say that control has passed from the top-level program to the function hello. After Steps 5 and 6 print out two lines, at Step 7, control will be passed back to the point after where the invocation was started. At Step 8, that has happened.
It is a common mistake for beginners to forget their parenthesis after the function name. This is particularly common in the case where there are no parameters required. Because the hello function defined above does not require parameters, it’s easy to forget the parenthesis. This is less common, but still possible, when trying to call functions that require parameters.
While some functions do calculate values, the python idea of a function is slightly different from the mathematical idea of a function in that not all functions calculate values. Consider, for example, the turtle functions in this section. They made the turtle draw a specific shape, rather than calculating a value.
While functions are not required, they help the programmer better think about the solution by organizing pieces of the solution into logical chunks that can be reused.
All Python programs must be written using functions
In the first several chapters, you have seen many examples of Python programs written without the use of functions. While writing and using functions is desirable and essential for good programming style as your programs get longer, it is not required.
