Foundations of Python Programming Functions First

Defining a new function does not make the function run. To execute the function, we need a function call. This is also known as a function invocation.

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.

Let’s take a closer look at what happens when you define a function and when you execute the function. Try stepping through the code above.

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 object of the ’function’ class. 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’.

Line 6 is just there to remind you of the difference between referring to the variable name (function name) hello and referring to the string “hello”. When we print a string’s type, we see it is of the ’str’ class.

At Step 4 we get to line 8, which has an invocation of the function. When a function is called (or invoked by a line of code, the lines of code inside the function’s body are executed in the usual way, but at the end of the function’s code block, the program’s execution jumps back to line of code just after the function invocation.

You can see that by following the next few steps. At Step 5, the green arrow indicates the ’line that just executed’, aka the function’s invocation, and the red arrow shows us the ’next line to execute’ which has moved to the function. We say that control has passed from the top-level program (not ’top’ as in the ’top of the code’ but as in top-hierarchial level) to the function hello. After Steps 5 through 7 print out two lines, at Step 8, control will be passed back to the point after where the invocation was started. By Step 9, that has happened.

The same process of invocation occurs again on line 10, with the function’s body (lines 2 and 3) getting executed a second time.

Check your understanding

def hello() -> None:
  print("Hello")
  print("Glad to meet you")

def hello() -> None:
  print("Hello")
  print("Glad to meet you")

hello()
print("It works")
hello()
hello()