Modules

The Python standard library

So far, we've only used functions that are built into Python, and that you can use straight away when you start a Python interpreter. However, you can import modules to get additional functionality. Many common modules are included in Python by default so that you don't need to install them—but you do have to import them! These modules make up the Python standard library:

• https://docs.python.org/3/library/index.html

import: importing modules

Let's consider the random module:

• https://docs.python.org/3/library/random.html

The random module contains many functions for randomization. For example, random.choice() takes an iterable, such as a list, and returns a single, randomly selected element. To use random.choice(), you need to import the random module.

# Preferred
import random
prime_numbers = [1, 3, 5, 7, 11]
print(random.choice(prime_numbers))

Output:

3

In the example above, you import the entire random module and then selectively call random.choice(). However, the import statement is flexible and allows you to import modules and functions in many different ways. These all have their place, but as a general rule, the import method shown above is clearest and therefore preferred.

You can also selectively import the choice() function. The downside of this way of importing is that you cannot easily tell anymore that choice() is part of the random module.

# Not preferred
from random import choice
print(choice(prime_numbers))

Output:

11

You can also directly import everything from the random module: a wildcard import. This is generally considered bad practice, because it makes it difficult to tell where a function comes from.

# Not preferred
from random import *
print(choice(prime_numbers))

Output:

7

Finally, the import statement allows you to rename the imported modules or functions. Again, this is generally not preferred because of potential confusion. Exceptions are libraries such as numpy for which an import-and-rename (import numpy as np) is so common that it is unlikely to result in confusion.

# Not preferred
import random as rnd
print(rnd.choice(prime_numbers))

Output:

11

Installing additional modules with pip

There are also many modules that are not part of the Python standard library, that is, modules that are not included with Python by default. Fortunately, it's easy to install these additional modules with pip, the __P__ackage __I__nstaller for __P__ython. pip installs packages from the Python Package Index, or PyPI, which is like an app-store for Python packages. Almost every Python package that exists is available from there:

• https://pypi.org/

pip is a command-line that is included with Python. If you're using a code editor with a Python environment, such as Rapunzel or Spyder, the easiest to call pip is simply by entering a pip command in the Python console. (This works because the code editor recognizes that a pip command is not Python code, but rather a call to the pip application.)

To install a package, simply execute pip install followed by the name of the package. For example, to install DataMatrix, a package for working with tabular data, which you may meet later in the data-science course, run:

pip install datamatrix

Review exercises

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