Libraries

Last updated on 2026-10-21 | Edit this page

Estimated time: 40 minutes

Overview

Questions

  • How can I use software that other people have written?
  • How can I find out what that software does?

Objectives

  • Explain what software libraries are and why programmers 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 or module is a collection of files that contain functions for use by other programs. They can also contain constants like Pi and Tau.

The Python standard library is a suite of modules that come with all Python installations.

You have additional libraries available to you from Anaconda and from package managers like PyPi.

Callout

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.

A program must import a library module before using it.

To use a library module you must import it. You only need to import a module once per file or interactive session.

Let’s use the math module to find the cosine of pi. This is a module from Python’s standard library.

PYTHON 

import math 
print("Pi is", math.pi) # Prints the constant pi stored in the math library

OUTPUT 

pi is 3.141592653589793

You can use imported functions using dot-notation. Prefix the function with the name of the library (math), add a `.’, then call it just as you would any function.

PYTHON 

x = math.cos(math.pi) # Assigns x the value returned by cos(pi)
print(x)

OUTPUT 

-1.0

Use help to learn about the contents of a library module.

  • This just like help for a function. It lists all the functions and constants available in the module.

PYTHON 

help(math)

OUTPUT 

Help on module math:

NAME
    math

MODULE REFERENCE
    http://docs.python.org/3/library/math

    The following documentation is automatically generated from the Python
    source files.  It may be incomplete, incorrect or include features that
    are considered implementation detail and may vary between Python
    implementations.  When in doubt, consult the module reference at the
    location listed above.

DESCRIPTION
    This module is always available.  It provides access to the
    mathematical functions defined by the C standard.

FUNCTIONS
    acos(x, /)
        Return the arc cosine (measured in radians) of x.
⋮ ⋮ ⋮

Import specific items from a library module to shorten programs.

  • Use from ... import ... syntax to load only specific items from a library module.
  • In this format, you don’t have to use the library name as a prefix.

PYTHON 

from math import pi, pow

radius = 5
area = pi * pow(5, 2)
print("The area of a circle with radius 5 is", area)

OUTPUT 

The area of a circle with radius 5 is 78.53981633974483

Create an alias for a library module when importing it to shorten programs.

  • Use import ... as ... to give a library a short alias while importing it.
  • You can refer to functions or constants in the library using that shortened name.

PYTHON 

import math as m

print('The cube root of 27 is', m.cbrt(27))

OUTPUT 

The cube root of 27 is 3.0

This is commonly used for libraries that are frequently used and/or have long names. For example, the library pandas, used later in this workshop, is aliased pd.

Challenge

Exploring the Math Module

  1. What function from the math module can you use to calculate a square root without using sqrt?
  2. Since the library contains this function, why does sqrt exist?

HINT: A square root can be written as x1/2 or x0.5 in exponent notation.

Using help(math) we see that we’ve got pow(x,y) in addition to sqrt(x), so we could use pow(x, 0.5) to find a square root too! The sqrt(x) function is more readable than pow(x, 0.5). Readability is a cornerstone of good programming, so it makes sense to provide a special function for this common case.

Challenge

Rewriting Absolute Value

Remember our previous absolute value exercise? The math module has a .fabs() function that returns the absolute value of a number.

Update the following code to simplify the process of computing the absolute value of each number in a list of numbers.

PYTHON 

from math import fabs
original = [-1.5, 0.2, 0.4, -1.3]
result = [] # Initialize an empty list
for value in original:
  result.append(____(value)) # Update this line
print(result)

PYTHON 

from math import fabs
original = [-1.5, 0.2, 0.4, -1.3]
result = [] # Initialize an empty list
for value in original:
  result.append(fabs(value)) 
print(result)

OUTPUT 

[1.5, 0.2, 0.4, 1.3]
Challenge

Getting Help with Modules

When a colleague of yours types help(statistics), Python reports an error:

ERROR 

NameError: name 'statistics' is not defined

What has your colleague forgotten to do?

Your colleague has forgotten to import the statistics module. This can be fixed as follows.

PYTHON 

import statistics
help(statistics)

OUTPUT 

Help on module statistics:

NAME
    statistics - Basic statistics module.
Challenge

Applying New Modules

Let’s say you want find to the average of the following five measurements in inches: 60, 65, 58, 70, 63. The statistics module should be able to help you.

  1. Create a list named heights with the the measurements [60, 65, 58, 70, 63].
  2. Find a function inside the statistics module that computes the average of a list. Use help() to find it.
  3. Apply that function to heights.

PYTHON 

import statistics
heights = ____

print("The average is", _____.______(heights), "inches.")

The function you’re looking for is mean, which computes the average of a list of numbers.

PYTHON 

import statistics
heights = [60, 65, 58, 70, 63]
print("The average is", statistics.mean(heights), "inches.")

OUTPUT 

The average is 63.2 inches.
Challenge

Importing With Aliases

  1. Fill in the blanks so that the program below prints the square root of pi.
  2. Rewrite the program so that it uses import without as.
  3. Which form do you find easier to read?

PYTHON 

import ____ as m
root_pi = ____.sqrt(___.pi)
print(____)

PYTHON 

import math as m
root_pi = m.sqrt(m.pi)
print(root_pi)

This can be rewritten as:

PYTHON 

import math
root_pi = math.sqrt(math.pi)
print(root_pi)

Since you just wrote the code, you might actually find the first version easier to read. But when trying to read a huge piece of code written by someone else, non-abbreviated names are often easier, except where there are clear conventions.

Challenge

Reading Error Messages

  1. Read the code below and try to identify what the errors are without running it.
  2. Run the code, and read the error message. What type of error is it?

PYTHON 

from math import log
log(0)

OUTPUT 

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1-d72e1d780bab> in <module>
      1 from math import log
----> 2 log(0)

ValueError: math domain error
  1. The logarithm of x is only defined for x > 0, so 0 is outside the domain of the function.
  2. You get an error of type ValueError, indicating that the function received an inappropriate argument value. The additional message “math domain error” makes it clearer what the problem is.
Key Points
  • Most of the power of a programming language is in its libraries.
  • A program must import a library module in order to use it.
  • Use help to learn about the contents of a library module.
  • The math and statistics modules contain helpful functions and constants for mathematical operations.
  • Import specific items from a library or create an alias to shorten programs.