** Function **is also the most used element in

**Function**combine variable, data, and operator. The function is one of the most used element because it will reduce working time

If we familiar about function in math and spreadsheet, you can understand easily.

The spreadsheet gives us a ready use function. For example average(A1

In mathematics, a **function** show of how a varying quantity of variable depends on another quantity. … If the f is a name of ** function** , the notation is y = f (x) (read f of x).

Python has many functions. There are a built-in function in python, functions in modules, and we can make new Built-in.

## Built-in Function

A built-in function is a function that Python already provides if we install it. We can see the built-in function here.

Then, to call

```
>>> x =[2,4,5,6]
>>> sum(x)
17
>>> len(x)
4
```

“

## Function in Standard Library

Python also has a hidden function at modules in the standard library folder. We have to call its modules to use it. This is

```
>>> import random
>>> random.randint(0, 20)
7
>>> random.randint(500, 50000)
18601
```

We can read the notation is

This is list of modules standard libray.

- os
- shutil
- glob
- sys
- re
- math
- urllib2
- smtplib
- date
- zlib
- Timer
- unittest

## Third Party Library

As I talk in the introduction, Python

`pip install modulename`

After install third party modules successfully, we can import to use it.

`import modulename`

modulename is name of modules that you want to install. Dont type module name. If you install BeautifulSoup modules, you can type pip install BeautifulSoup

`pip install BeautifulSoup`

## User Defined Function

If we want to make our own function, we can make it in python.

Syntax to create our create from scratch function is:

```
def NAME( PARAMETERS ):
STATEMENTS
```

For example, if we want to create average function. We can make function like in code below.

```
>>>def average(xx):
av = sum(xx)/len(xx)
return av
y = [1,2,3,4,5]
print average(y)
```

That is example of user-defined function.