Libraries

Overview

Teaching: 10 min
Exercises: 10 min
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 create and use them.

  • Write programs that import and use libraries from Python’s standard library.

  • Find and read documentation for standard libraries interactively (in the interpreter) and online.

Most of the power of a programming language is in its (software) 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.

A program must import a library module before using it.

import math

print('pi is', math.pi)
print('cos(pi) is', math.cos(math.pi))
pi is 3.141592653589793
cos(pi) is -1.0

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

help(math)
Help on module math:

NAME
    math

MODULE REFERENCE
    http://docs.python.org/3.5/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(...)
        acos(x)

        Return the arc cosine (measured in radians) of x.
⋮ ⋮ ⋮

Import specific items from a library module to shorten programs.

from math import cos, pi

print('cos(pi) is', cos(pi))
cos(pi) is -1.0

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

import math as m

print('cos(pi) is', m.cos(m.pi))
cos(pi) is -1.0

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?

Solution

  1. Using help(math) we see that we’ve got pow(x,y) in addition to sqrt(x)
  2. The sqrt(x) function (like much of the math library) has it’s origins in C’s math library. Consequently, it might be somewhat faster than pow(x,y). Also, it might be more readable than pow(x, 0.5) when implementing equations. However, sqrt(x) doesn’t support negative arguments.

Locating the Right Module

You want to transform the format of date to the iso standard:

year = 2016
month = 10
day = 22
  1. Which standard library module could help you?
  2. Which function would you select from that module?
  3. Try to write a program that uses the function.

Solution

The datetime module seems like it could help you.

You could use date(year, month, date).isoformat() to convert your date:

import datetime

iso_date = datetime.date(year, month, day).isoformat()
print(iso_date)

or more compactly:

import datetime

print(datetime.date(year, month, day).isoformat())

Jigsaw Puzzle (Parson’s Problem) Programming Example

Rearrange the following statements so that a random DNA base is printed. Not all statements may be needed. Feel free to use/add intermediate variables.

bases="ACTTGCTTGAC"
import math
import random
len(bases)
len(bases)+1
math.floor(s1)
math.ceil(s1)
print("random base ",bases[])
random.random()*l

When Is Help Available?

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

NameError: name 'math' is not defined

What has your colleague forgotten to do?

Solution

Importing the math module (import math)

Importing With Aliases

  1. Fill in the blanks so that the program below prints 90.0.
  2. Rewrite the program so that it uses import without as.
  3. Which form do you find easier to read?
import math as m
angle = ____.degrees(____.pi / 2)
print(____)

Solution

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

can be written as

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

Since you just wrote the code and are familiar with it, you might actually find the first version easier to read. But when trying to read a huge piece of code written by someone else, or when getting back to your own huge piece of code after several months, non-abbreviated names are often easier, expect where there are clear abbreviation conventions.

There Are Many Ways To Import Libraries!

Match the following print statements with the appropriate library calls

Library calls:

  1. from math import sin,pi
  2. import math
  3. import math as m
  4. from math import *

Print commands:

  1. print("sin(pi/2) =",sin(pi/2))
  2. print("sin(pi/2) =",m.sin(m.pi/2))
  3. print("sin(pi/2) =",math.sin(math.pi/2))

Importing Specific Items

  1. Fill in the blanks so that the program below prints 90.0.
  2. Do you find this version easier to read than preceding ones?
  3. Why wouldn’t programmers always use this form of import?
____ math import ____, ____
angle = degrees(pi / 2)
print(angle)

Solution

from math import degrees, pi
angle = degrees(pi / 2)
print(angle)

Most likely you find this version easier to read since it’s less dense. The main reason not to use this form of import is to avoid name clashes. For instance, you wouldn’t import degrees this way if you also wanted to use the name degrees for a variable or function of your own. Or if you were to also import a function named degrees from another library.

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?
from math import log
log(0)

Solution

  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.

  • Import specific items from a library to shorten programs.

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