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 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 is an extensive suite of modules that comes with Python itself.
• Many additional libraries are available from 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.

A program must import a library module before using it.

• Use import to load a library module into a program’s memory.
• Then refer to things from the module as module_name.thing_name.
  • Python uses . to mean “part of”.
• Using math, one of the modules in the standard library:

import math

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

pi is 3.141592653589793
cos(pi) is -1.0

• Have to refer to each item with the module’s name.
  • math.cos(pi) won’t work: the reference to pi doesn’t somehow “inherit” the function’s reference to math.

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

• Works just like help for a function.

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.

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

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.

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

import math as m

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

cos(pi) is -1.0

• Commonly used for libraries that are frequently used or have long names.
  • E.g., matplotlib plotting library is often aliased as mpl.
• But can make programs harder to understand, since readers must learn your program’s aliases.

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), so we could use pow(x, 0.5) to find a square root.

2. The sqrt(x) function is arguably more readable than pow(x, 0.5) when implementing equations. Readability is a cornerstone of good programming, so it makes sense to provide a special function for this specific common case.

   Also, the design of Python’s math library has its origin in the C standard, which includes both sqrt(x) and pow(x,y), so a little bit of the history of programming is showing in Python’s function names.

Locating the Right Module

You want to select a random character from a string:

bases = 'ACTTGCTTGAC'

1. Which standard library module could help you?
2. Which function would you select from that module? Are there alternatives?
3. Try to write a program that uses the function.

Solution

The random module seems like it could help you.

The string has 11 characters, each having a positional index from 0 to 10. You could use random.randrange function (or the alias random.randint if you find that easier to remember) to get a random integer between 0 and 10, and then pick out the character at that position:

from random import randrange

random_index = randrange(len(bases))
print(bases[random_index])

or more compactly:

from random import randrange

print(bases[randrange(len(bases))])

Perhaps you found the random.sample function? It allows for slightly less typing:

from random import sample

print(sample(bases, 1)[0])

Note that this function returns a list of values. We will learn about lists in episode 11.

There’s also other functions you could use, but with more convoluted code as a result.

Jigsaw Puzzle (Parson’s Problem) Programming Example

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

bases="ACTTGCTTGAC"
import math
import random
___ = random.randrange(n_bases)
___ = len(bases)
print("random base ", bases[___], "base index", ___)

Solution

import math 
import random
bases = "ACTTGCTTGAC" 
n_bases = len(bases)
idx = random.randrange(n_bases)
print("random base", bases[idx], "base index", idx)

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, except where there are clear abbreviation conventions.

There Are Many Ways To Import Libraries!

Match the following print statements with the appropriate library calls.

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))

Library calls:

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

Solution

1. Library calls 1 and 4. In order to directly refer to sin and pi without the library name as prefix, you need to use the from ... import ... statement. Whereas library call 1 specifically imports the two functions sin and pi, library call 4 imports all functions in the math module.
2. Library call 3. Here sin and pi are referred to with a shortened library name m instead of math. Library call 3 does exactly that using the import ... as ... syntax - it creates an alias for math in the form of the shortened name m.
3. Library call 2. Here sin and pi are referred to with the regular library name math, so the regular import ... call suffices.

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.