# First Functions

Create a file called `my_functions.py` and take a look at the code below:

```python
def add_five(number):
    return number + 5

what_is_this_number = add_five(10) # It's a hard question but I guess 15...
print(what_is_this_number)
```

We're actually doing two things here: defining the `add_5` function and then calling it. We define the `add_five` function by first typing **def**, then the function name "add\_five", we then have brackets which contain _parameters_ and finally on that line a colon. The code of the function simply consists of the `return` keyword and a maths expresision that adds 5 to `number`.

That's a lot to take in, but it's simple. The parameters are simply inputs for the function. The colon says it's a new block of code i.e. all the code that's indented belongs to the function. The `return` keyword simply says what the function outputs. Let's see some other functions:

```python
# This function takes a string called 'message' and returns a new string
# that adds the word adios to the message
def say_goodbye(message):
    return message + ' ¡Adios!'

print(say_goodbye('Hi my name is Paul!'))

# Functions you defined can call other functions as well
def add_ten(number):
    with_five_more = add_five(number)
    return add_five(with_five_more)

print(add_ten(10)) # You'll get 20

# We can use the output of one function as input for another
def add_ten_version2(number):
    return add_five(add_five(number))

print(add_ten_version2(10)) # You'll get 20
```

So far our functions have been pretty much the same format: def keyword, function name, a parameter in brackets, some code that returns a value at the end. We got some flexibility, especially with the parameters and return values:

```python
# Yep, you can have multiple parameters
def my_multiply(x, y):
    return x * y

print(my_multiply(7, 5)) # 35

# Sometimes you don't need parameters
def greetings():
    return '¡Hola!'

print(greetings()) # You must still use the brackets even without parameters!

# Some functions may not return values
def haiku():
    print('Do I want to fight? Chuck Norris roundhouse kicked me! I will fight no more...')

haiku() # Because this function prints we don't call the print statement again
```

There's something to note about the `return` statement, it ends the execution of the function. So every line of code that comes after it is never run. Try this quick example:

```python
def maths_for_fun(num1, num2, num3):
    mult_nums = num1 * num3
    div_nums = num2 / num1
    return mult_nums - div_nums
    # This code with never run
    add_nums = num2 + num3
    return add_nums

print(maths_for_fun(5,25,6)) # 25 instead of 31
```
