17: Functions in python

The Theory:

Just like a procedure, a function is a section of code that you can (re)use as many times as you like. Just like a procedure, functions have names (which should describe what they do).

Functions and procedures are both sub programs (they allow you to split code up into smaller parts).

A function returns a value. A procedure doesn’t.

The difference between a function and a procedure

The example below displays a random greeting (such as a Yorkshire “Ey up”).

Lines 3-6 define the function (tells python what to do)

Line 9 calls the function (tells python to actually do it)

Line 6 returns a value from the function to the line of code which called the function

How a function works

Defining a function means telling python what your function is going to do. The function definition starts with def followed by the name (identifier) then the code that makes the function work.
Lines 3-6 define get_greeting

The identifier of a function is just the name of the function. It should describe what the procedure will do when you call it. The identifiers for functions have the same naming conventions (rules) as variables (use snake_case with underscores instead of spaces to describe what the procedure does)
The identifier of the function on line 4 is get_greeting because the function chooses a random greeting

Calling a function means telling python to actually run your function code. You call the function by writing the function name followed by round brackets( ) (sometimes called parenthesis).
Line 9 calls get_greeting (which makes lines 5-6 run)

Returning a value means sending a value (that the function has calculated) back to the code which called the function. The return value is like the output of a function.
Line 6 chooses a random greeting and then returns it back to line 9 so it can be stored into the variable random_greeting

Built-in functions

Some functions are built in to python which means you can use them without having to tell python how they work.

The example below uses three built-in functions:

input is a built-in function which asks a question and returns whatever the user types in. The return value for input is always a string (text)
Line 1 calls the function input and stores the return value into age_as_string

int is a built-in function which converts a string (text) to an integer (whole number) .
Line 2 calls the function int and assigns the return value to the variable age_in_denary

bin is a built-in function which converts a denary number (humans count in denary: 0-9) into binary (computers count in binary: 0-1)
Line 3 calls the function bin and assigns the return value to the variable age_in_binary

print is a built in procedure. It’s not a function because it doesn’t return a value.

All of the above are built-in sub programs because procedures and functions are both types of sub program

Built in sub program

Advantages of using functions

Being able to create and use your own functions and procedures is a real break-through moment when you’re learning to program.

Using functions and procedures is an example of abstraction because you don’t need to know how the function works in order to call it

Abstraction means hiding unnecessary detail to focus on the most important information you need to solve a problem.

Making your own functions is an example of decomposition because you can break the program up into smaller parts

Decomposition means breaking a large problem down into smaller parts so that each part can be solved separately, making the whole problem easier to solve.

Call stack

Every time your program calls a sub program (procedure or function), it needs to know where to continue once the sub program has finished running.

Python does this by using a call stack.

A stack is a data structure where the last item you put in (store) is the first item you take out (read)

A data structure stores multiple values

Think of a call stack like a stack of paper. Each piece of paper contains a job that you need to complete. Whenever you add a new job to the stack, that takes over and you have to finish that job. Once you’re done, you remove that piece of paper from the stack and you carry on from where you left off.

Page 1: Intro

Page 2: The theory: learn what you need to know as fast as possible.

Page 3: Try it: try out and adapt some working python code snippets.

Page 4: Debug it: Learn how to find and fix common mistakes.

Page 5: Extend it: Choose a project idea to use your newfound python skills.