Python Libraries¶

Objectives¶

Explain what software libraries are and why programmers create and use them.
Write programs that import and use modules from Python's standard library.
Find and read documentation for the standard library interactively (in the interpreter) and online

Most of the power of a programming language is in its libraries.¶

A library is a collection of files (called modules) that contains functions for use by other programs.
- May also contain data values (e.g., numerical constants) and other things.
- Library's contents are supposed to be related, but there's no way to enforce that.
The Python [standard library][stdlib] is an extensive suite of modules that comes with Python itself.
Many additional libraries are available from [PyPI][pypi] (the Python Package Index).
We will see later how to write new libraries.

Libraries and modules¶

A library is a collection of modules, but the terms are often used interchangeably, especially since many libraries only consist of a single module, so don't worry if you mix them.

In [2]:

import math

print('pi is', math.pi)
print('cos(pi) is', math.cos(math.pi))

pi is 3.141592653589793
cos(pi) is -1.0

Import specific items from a library module to shorten programs.¶

Use from ... import ... to load only specific items from a library module.
Then refer to them directly without library name as prefix.

In [3]:

from math import cos, pi

print('cos(pi) is', cos(pi))

cos(pi) is -1.0

Create an alias for library¶

Use import ... as ... to give a library a short alias while importing it.
Then refer to items in the library using that shortened name.

In [10]:

import math as m

print('cos(pi) is', m.cos(m.pi/4))

cos(pi) is 0.7071067811865476

In [9]:

import numpy as np

print('cos(pi) is', np.cos(m.pi/4))

cos(pi) is 0.7071067811865476

Use with caution¶

Aliases can make programs harder to understand, since readers must learn your program's aliases.

Excercise¶

Find a standard library that can help you select a random character from a string.
Try writing a code snippet that uses a function from a library that selects a random character from the string

bases = 'ACTTGCTTGAC'

Note: You may use AI assistance, but you should be able to explain what the code does!

In [ ]:

import random

random.choice(

Fill in the blanks so that the program below prints 90.0

import math as m
angle = ____.degrees(____.pi / 2)
print(____)