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Learn how modules work in the Python programming language.

Modules in Python

Let's say you are writing software that handles monetary transactions. If you used Python's built-in [floating-point arithmetic](https://en.wikipedia.org/wiki/Floating-point_arithmetic) to calculate a sum, it would result in a weirdly formatted number. 

```py
cost_of_gum = 0.10
cost_of_gumdrop = 0.35

cost_of_transaction = cost_of_gum + cost_of_gumdrop
# Returns 0.44999999999999996
```

Being familiar with rounding errors in floating-point arithmetic you want to use a data type that performs decimal arithmetic more accurately. You could do the following:

```py
from decimal import Decimal

cost_of_gum = Decimal('0.10')
cost_of_gumdrop = Decimal('0.35')

cost_of_transaction = cost_of_gum + cost_of_gumdrop
# Returns 0.45 instead of 0.44999999999999996
```

Above, we use the `decimal` module's `Decimal` data type to add 0.10 with 0.35. Since we used the `Decimal` type the arithmetic acts much more as expected.

Usually, modules will provide functions or data types that we can then use to solve a general problem, allowing us more time to focus on the software that we are building to solve a more specific problem.

Ready, set, fix some floating point math by using decimals!
 


Modules Python Decimals

Notice that when we want to invoke the `randint()` function we call `random.randint()`. This is default behavior where Python offers a _namespace_ for the module. A namespace isolates the functions, classes, and variables defined in the module from the code in the file doing the importing. Your _local namespace_, meanwhile, is where your code is run.

Python defaults to naming the namespace after the module being imported, but sometimes this name could be ambiguous or lengthy. Sometimes, the module's name could also conflict with an object you have defined within your local namespace. 

Fortunately, this name can be altered by _aliasing_ using the `as` keyword:

```py
import module_name as name_you_pick_for_the_module
```

Aliasing is most often done if the name of the library is long and typing the full name every time you want to use one of its functions is laborious.

You might also occasionally encounter `import *`. The `*` is known as a "wildcard" and matches anything and everything. This syntax is considered dangerous because it could _pollute_ our local namespace. Pollution occurs when the same name could apply to two possible things. For example, if you happen to have a function `floor()` focused on floor tiles, using `from math import *` would also import a function `floor()` that rounds down floats.

Let's combine your knowledge of the `random` library with another fun library called `matplotlib`, which allows you to plot your Python code in 2D. 

You'll use a new `random` function `random.sample()` that takes a range and a number as its arguments. It will return the specified number of random numbers from that range.
```py
#random.sample takes a list and randomly selects k items from it
new_list = random.sample(list, k)
#for example:
nums = [1, 2, 3, 4, 5]
sample_nums = random.sample(nums, 3)
print(sample_nums) # 2, 5, 1
```



Modules Python Namespaces

You may remember the concept of _scope_ from when you were learning about functions in Python. If a variable is defined _inside_ of a function, it will not be accessible _outside_ of the function. 

Scope also applies to *classes* and to the *files* you are working within.

_Files_ have scope? You may be wondering.

Yes. Even files inside the _same directory_ do not have access to each other's variables, functions, classes, or any other code. So if I have a file **sandwiches.py** and another file **hungry_people.py**, how do I give my hungry people access to all the sandwiches I defined?

Well, _*files are actually modules*_, so you can give a file access to another file's content using that glorious `import` statement.

With a single line of `from sandwiches import sandwiches` at the top of **hungry_people.py**, the hungry people will have all the sandwiches they could ever want.



Modules Python Files and Scope


In the world of programming, we care a lot about making code reusable. In most cases, we write code so that it can be reusable for ourselves. But sometimes we share code that's helpful across a broad range of situations.

In this lesson, we'll explore how to use tools other people have built in Python that are not included automatically for you when you install Python. Python allows us to package code into files or sets of files called [_modules_](https://www.codecademy.com/resources/docs/python/modules?page_ref=catalog). 

A module is a collection of Python declarations intended broadly to be used as a tool. Modules are also often referred to as "libraries" or "packages" — a package is really a directory that holds a collection of modules.

Usually, to use a module in a file, the basic syntax you need at the top of that file is:

```py
from module_name import object_name
```

Often, a library will include a lot of code that you don't need that may slow down your program or conflict with existing code. Because of this, it makes sense to only import what you need. 

One common library that comes as part of the Python Standard Library is [`datetime`](https://www.codecademy.com/resources/docs/python/dates?page_ref=catalog). `datetime` helps you work with dates and times in Python. 

Let's get started by importing and using the `datetime` module. In this case, you'll notice that `datetime` is both the name of the library _and_ the name of the object that you are importing.



Modules Python Introduction


You've learned:
- what modules are and how they can be useful
- how to use a few of the most commonly used Python libraries
- what namespaces are and how to avoid polluting your local namespace
- how scope works for files in Python

Programmers can do great things if they are not forced to constantly reinvent tools that have already been built. With the power of modules, we can import any code that someone else has shared publicly. 

In this lesson, we covered some of the Python Standard Library, but you can explore all the modules that come packaged with every installation of Python at the Python Standard Library documentation. 

This is just the beginning. Using a package manager (like conda or pip3), you can install any modules available on the Python Package Index.

The sky's the limit!



Modules Python Review

`datetime` is just the beginning. There are hundreds of Python modules that you can use. Another one of the most commonly used is [`random`](https://www.codecademy.com/resources/docs/python/random-module?page_ref=catalog) which allows you to generate numbers or select items at random.

With `random`, we'll be using more than one piece of the module's functionality, so the import syntax will look like:

```py
import random
```

We'll work with two common `random` functions:
- [`random.choice()`](https://www.codecademy.com/resources/docs/python/random-module/choice?page_ref=catalog) which takes a list as an argument and returns a number from the list
- [`random.randint()`](https://www.codecademy.com/resources/docs/python/random-module/randint?page_ref=catalog) which takes two numbers as arguments and generates a random number between the two numbers you passed in

Let's take randomness to a whole new level by picking a random number from a list of randomly generated numbers between 1 and 100.

