Contents

Working with libraries¶

##3 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][stdlib] is an extensive suite of modules that comes with Python itself.

  • Many additional libraries are available from [PyPI][pypi] (the Python Package Index).

  • We will see later how to write new libraries.

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][stdlib] is an extensive suite of modules that comes with Python itself.

  • Many additional libraries are available from [PyPI][pypi] (the Python Package Index).

  • We will see later how to write new libraries.

Note

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)

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.

  • But can make programs harder to understand, since readers must learn your program’s aliases.

Exercise: 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?

See 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.

Exercise: Locating the right Module

You want to select a random character from a string:

bases = 'ACTTGCTTGAC'

  1. Which [standard library][stdlib] 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.

See Solution

The [random module][randommod] 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])

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

Running someone else’s Python script Reading data with pandas