Libraries
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Estimated time 20 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 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.
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:
PYTHON
import math
print('pi is', math.pi)
print('cos(pi) is', math.cos(math.pi))
OUTPUT
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 topi
doesn’t somehow “inherit” the function’s reference tomath
.

Use help
to learn about the contents of a library
module.
 Works just like help for a function.
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 ...
to load only specific items from a library module.  Then refer to them directly without library name as prefix.
PYTHON
from math import cos, pi
print('cos(pi) is', cos(pi))
OUTPUT
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.
PYTHON
import math as m
print('cos(pi) is', m.cos(m.pi))
OUTPUT
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 asmpl
.
 E.g., the
 But can make programs harder to understand, since readers must learn your program’s aliases.
Using
help(math)
we see that we’ve gotpow(x,y)
in addition tosqrt(x)
, so we could usepow(x, 0.5)
to find a square root.The
sqrt(x)
function is arguably more readable thanpow(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:
PYTHON
= 'ACTTGCTTGAC' bases
 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.
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:
PYTHON
from random import randrange
= randrange(len(bases))
random_index print(bases[random_index])
or more compactly:
PYTHON
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:
PYTHON
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:
PYTHON
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.
PYTHON
="ACTTGCTTGAC"
basesimport math
import random
= random.randrange(n_bases)
___ = len(bases)
___ print("random base ", bases[___], "base index", ___)
PYTHON
import math
import random
= "ACTTGCTTGAC"
bases = len(bases)
n_bases = random.randrange(n_bases)
idx print("random base", bases[idx], "base index", idx)
Importing the math module (import math
)
PYTHON
import math as m
= m.degrees(m.pi / 2)
angle print(angle)
can be written as
PYTHON
import math
= math.degrees(math.pi / 2)
angle 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, nonabbreviated 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 *
 Library calls 1 and 4. In order to directly refer to
sin
andpi
without the library name as prefix, you need to use thefrom ... import ...
statement. Whereas library call 1 specifically imports the two functionssin
andpi
, library call 4 imports all functions in themath
module.  Library call 3. Here
sin
andpi
are referred to with a shortened library namem
instead ofmath
. Library call 3 does exactly that using theimport ... as ...
syntax  it creates an alias formath
in the form of the shortened namem
.  Library call 2. Here
sin
andpi
are referred to with the regular library namemath
, so the regularimport ...
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.
PYTHON
from math import degrees, pi
= degrees(pi / 2)
angle 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.
OUTPUT

ValueError Traceback (most recent call last)
<ipythoninput1d72e1d780bab> in <module>
1 from math import log
> 2 log(0)
ValueError: math domain error
 The logarithm of
x
is only defined forx > 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.
Keypoints
 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.