Introduction to Functions

What are functions?

Let's forget about Python for a moment and go back a few years all the way back to Algebra. Do you remember any expressions that resemble the form of f(x) = x + 3?

f(x) = x + 3 is a function. For any given x, the function will add 3 to x and return that value.

• f(5) = 5 + 3, returns 8
• f(10) = 10 + 3, returns 13
• f(-3) = -3 + 3, returns 0

In mathematical terms, a function takes one or more inputs and produces an output. Our previous function f(x) = x + 3 takes x as an input and outputs x + 3.

f in front of the parentheses is an arbitrary name used to represent a function. We can choose to rename our function to add_three. This changes our function expression to add_three(x) = x + 3.

A function can also take more than one input (e.g., add_three(a, b) = a + b).

In programming, an inputs is referred as an argument, and an output is referred as a return value.

Slicing a List

In Python, you can easily slice and retrieve a subset of a list using the slice notation. The basic syntax is my_list[start:stop] where my_list is a list-like value (or a variable). start and stop specify the index where you would like to start/stop retrieving values.

Variable Scopes

Any variable that is declared outside of the function is in the global scope. A variable in the global scope is called a global variable. Both global_one and global_two variables in the code below are examples of global variables.

Variable Scopes

Any variable that is used inside a function is in the function scope. A variable in the function scope is called a local variable. When a function is called, a function scope for that function is created. The function scope includes both the arguments of the function and any variables created inside the function.

Python throws an error 🚫 since x is already destroyed! Remember, function arguments reside in the local environment of the function and are destroyed once the function finishes running.