## Functions

You have already seen and used the functions in the previous parts of the course. It is called a function call whenever we have used the following expression:

*function_name()*

For example, there are some features that we have already seen before

- print(“Hello World”)
- range(10)
- max()

The word that precedes the parentheses is the **function name**, while the values or variables within the brackets are called **arguments**.

## Function definition

So far you have seen and worked with already defined functions, but functions can be created using def.

You define a simple function that prints the first 5 numbers of the Fibonacci sequence

```
def fibonacci():
i = 1
n1 = 0
n2 = 1
print ( n1 + n2)
while i < 10:
n = n1 + n2
print( n )
n1 = n2
n2 = n
i += 1
fibonacci()
```

Executing the function you will obtain the sequence of the first 10 numbers of the Fibonacci sequence:

```
>>>
1
1
2
3
5
8
13
21
34
55
```

**⇐ Go to Python Lesson 3.1 – Functions and modules – Code Reuse**