# Libraries

## Overview

Teaching:10 min

Exercises:10 minQuestions

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 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 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”.

- Python uses
- 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/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 ...`

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., the
`matplotlib`

plotting library is often aliased as`mpl`

.

- E.g., the
- But can make programs harder to understand, since readers must learn your program’s aliases.

## Exploring the Math Module

- What function from the
`math`

module can you use to calculate a square rootwithoutusing`sqrt`

?- Since the library contains this function, why does
`sqrt`

exist?## Solution

- 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.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'`

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

The random module seems like it could help.

The string has 11 characters, each having a positional index from 0 to 10. You could use the

`random.randrange`

or`random.randint`

functions to get a random integer between 0 and 10, and then select the`bases`

character at that index:`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 but might be a bit harder to understand just by reading:`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.

The simplest and shortest solution is the

`random.choice`

function that does exactly what we want:`from random import choice print(choice(bases))`

## 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

- Fill in the blanks so that the program below prints
`90.0`

.- Rewrite the program so that it uses
`import`

without`as`

.- 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:

`print("sin(pi/2) =", sin(pi/2))`

`print("sin(pi/2) =", m.sin(m.pi/2))`

`print("sin(pi/2) =", math.sin(math.pi/2))`

Library calls:

`from math import sin, pi`

`import math`

`import math as m`

`from math import *`

## Solution

- 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.- 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`

.- Library call 2. Here
`sin`

and`pi`

are referred to with the regular library name`math`

, so the regular`import ...`

call suffices.

Note:although library call 4 works, importing all names from a module using a wildcard import is not recommended as it makes it unclear which names from the module are used in the code. In general it is best to make your imports as specific as possible and to only import what your code uses. In library call 1, the`import`

statement explicitly tells us that the`sin`

function is imported from the`math`

module, but library call 4 does not convey this information.

## Importing Specific Items

- Fill in the blanks so that the program below prints
`90.0`

.- Do you find this version easier to read than preceding ones?
- Why
wouldn’tprogrammers 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

- Read the code below and try to identify what the errors are without running it.
- Run the code, and read the error message. What type of error is it?
`from math import log log(0)`

## Solution

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

- The logarithm of
`x`

is only defined for`x > 0`

, so 0 is outside the domain of the function.- 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.