Python Tutorial #2

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Python: classes, modules and packages

Netsoc

Stephen Shaw

2010

Netsoc (Stephen Shaw) <[email protected]> 2010 1 / 25

Getting started

SSH to one of our servers$ ssh [email protected]: Enter login.netsoc.tcd.ie as the hostnameNX on cube if you want - all you need is a shell though

No netsoc account?CS: macneill.scss.tcd.ieMaths servers?Talk to an admin before you leave so you have an account for next time


macneill.scss.tcd.ie

The plan

ScheduleThis week - Classes, modules, packages, regular expressions,documentation, the PyPIWeek 3 - Miscellaneous fun with scientific computing

These slides: http://www.netsoc.tcd.ie/~stesh/python/2.pdf


http://www.netsoc.tcd.ie/~stesh/python/2.pdf

Object-orientation

Python is very object-oriented (maybe even more than Java?)

Nearly everything is an object

Numbers are objects:

1 >>> from math import pi2 >>> pi.is_integer ()3 False

Classes are objects

Multiple inheritance - add as many superclasses as you want


Our first class

A silly example:

1 c l a s s Student(object):2 ''' Represents a student '''3 def __init__(self , name , age , course):4 self.name = name5 self.age = age6 self.course = course7

8 def do_assignment(self , assignment):9 for section in assignment:

10 section.complete_dilligently ()11

12 c l a s s Programmer(student):13 ''' Represents a programmer '''14 def __init__(self , name , age , course=CS):15 super(student , self).__init__(name , age , course):16 self.lives_with_parents = True17

18 def do_assignment(self , assignment):19 i f assignment.deadline () == today:20 for section in assignment:21 section.do_hurriedly ()


Our first class

__init__ is the constructor

self refers to the current object

Have to pass self as an argument to method definitions:

1 def is_loser(self):2 return self.age > 40 and self.lives_with_parents ()

call methods like so:

1 >>> bob = Programmer("Bob", 23)2 >>> bob.is_loser ()3 False


Modules

Organized bits of code that add functionality to Python

What is module?

a file with extension .py

That's it!

Even the tiniest script is a moduleThousands of modules out there:

CGI, MySQL, regular expressions, scientific computing(scipy, numpy, sympy), graphics(PyGraph), GUIs (PyGTK), linguistics1

(nltk)

We'll look at a few of these today

1Obligatory plugNetsoc (Stephen Shaw) <[email protected]> 2010 7 / 25

Modules

Subdivide your code into useful functions, etc.

Define a function called main() (or anything else that pleases you)

1 i f __name__ == '__main__ ':2 main()

Only execute the main() function if the module is being run as a script


Modules

1 #!/usr/bin/env python32

3 nato = '''alpha bravo charlie delta echo foxtrot golf hotel india �juliet

4 kilo lima mike november oscar papa quebec romeo sierra tango5 uniform vector whiskey x-ray yankee zulu '''.split ()6

7

8 def phon(letter , alphabet=nato):9 ''' Return the phonetic representation of letter in alphabet '''

10 return {x[0]:x for x in alphabet }[ letter]11

12 def transcribe(utterance , alphabet=nato):13 ''' Return utterance transcribed in the given phonetic alphabet �

'''14 return ' '.join(phon(c, alphabet) for c in utterance.lower () i f c.�

isalpha ())15

16 def main():17 pr int (transcribe(input('utterance: ')))18

19 i f __name__ == '__main__ ':20 main()


Modules

to access a module, use the import keyword

import math - give me access to mathematical functions and constants

Now we can treat math like an object with methods:

1 >>> math.cos(math.pi)2 -1.03

4 >>> math.log (1)5 0.0

Can also import specific things: from math import degrees

We'll see this in action with the re module, and learn about regularexpressions in the process


Regular expressions for mathematicians

Formal language theory

Mathematicians and computational linguists use regular expressions todefine regular sets

The same expressive power as regular grammmars

All regular expressions have a generatively-equivalent finite-stateautomaton

Given a finite alphabet Σ, the empty set ∅, and the empty string ϵ, wedefine the closure operators of concatenation, alternation (|),Kleene-plus (+) and Kleene-star (∗) as follows. . .

. . .Who cares?


Regular expressions for programmers

Programmers use regular expressions to match strings in text'In this file, how many times does the letter a follow two or moreoccurences of a number divisible by 3?''Is the email address this user just gave me valid?'

Can also capture matched strings and replace them'Change all the semicolons to apostrophes''Please sanitize this $_POST so my webapp doesn't get pwned'

Not defined in python's syntax (c.f. perl, ruby)

Probably a good thing:

1 die unless chomp($line) =˜ s /(\ s +\w+(<\w+>)?,?)�{1,});/gi;

Made available in the re module instead


Some regular expressions

Expression What it recognizesa a single occurrence of a. a single occurrence of any character

a* zero or more occurrences of aa+ one or more occurrences of a

a|b a single occurrence a or of b (but not both)ab a single a followed by a single b

ab? a single a, optionally followed by a single b(ab){1-3} one, two or three occurences of ab

ˆ[a-zA-Z0-9]+.*[4-9]?$ one or more alphanumeric characters at thestart of a string, followed by any number ofany character, optionally termined by someinteger between 4 and 9 inclusive


Who is logged in whose username begins with a vowel?

1 #!/usr/bin/env python3 .12

3 import re4 import os5

6 begins_with_vowel = re.compile('ˆ[ AEIOUaeiou ].*$')7

8 people_online = os.popen('who')9 for person in people_online:

10 person = person.split(' ')[0]11 i f re.match(begins_with_vowel , person):12 pr int (person)


Show me all the links on some website

1 #!/usr/bin/env python32

3 import re # for regular expressions4 from sys import argv , stderr , exit # for system operations5 import urllib.request # for deailing with http requests6

7 # Get the URL from the command line8 website = exit("Give me a URL!") i f len(argv) < 2 else argv [1]9

10 # request data from the remote host11 response = urllib.request.urlopen(website)12

13 # read the response and decode it to Unicode14 data = response.read()15 html = data.decode('utf -8')16

17 # I've probably got this wrong18 urls = re.compile('[a-z][\w -]+:/{1 ,3}.[a-z\d.\ -]+[.][a-z]{2 ,4}/?')19

20 # find them all and print them21 for link in re.findall(urls , html):22 pr int (link)


Know your limits

Regular expressions are good atBeing flexibleBeing fast

Regular expressions are bad atNatural languagesMost programming languagesHTMLAnything which relies on bracket balancing

You can prove that this is the case


Documentation

Programmers like documentation

Usually a pain

As painless as possible in python


Documentation

1 /**2 * Generate a random list of ShoppingItems from the given list ,3 * of the given size.4 *5 * @param items6 * @param desiredSize7 * @return A ShoppingList object containing the given number of �

items8 * @throws InsufficientItemsException if the given size is less9 * than the total number of items available in the given list

10 */11 protected ShoppingList getShoppingList(List <ShoppingItem > items , �

i n t12 desiredSize);

No.


Documentation

Python is self-documenting (almost)

1 def factorial(n):2 ''' Return the factorial of n. This implementation is recursive3

4 Arguments:5 n -- a non -negative integer6

7 Exceptions:8 ValueError -- if n is negative9

10 >>> factorial (7)11 504012

13 >>> [factorial(n) for n in range (10)]14 [1, 1, 2, 6, 24, 120, 720, 5040, 40320 , 362880]15

16 '''17 i f n == 0:18 return 119 else :20 return n * factorial(n - 1)


Documentation

A multiline string just after a class or function definition

Called a docstring

Documentation there for you when you're writing code

Call help(thing) to generate nice formatted documentation

pydoc somemodule on the command line generates documentation fora module or package

A package is just a collection of modules

pydoc -p someport makes a nice web interface to your documentation

Just about everything in python is documented in this way


Doctests

This is really cool

Include example usage in function/method docstrings

Python can check if they do what you expect them to do

Like a baby unit test

It's common to end a module with

1 i f __name__ == '__main__ ':2 import doctest3 doctest.testmod ()

Run the module as a script to test it


The PyPI

Python Package Index

The source for python software

Thousands of packages and modules

http://pypi.python.org/pypi

Installing a package: pip install <package>


http://pypi.python.org/pypi

2 or 3?

Python is in a bit of a crisis at the moment

Python 3 is the latest release

Deliberately not backwards-compatible

Cleaning up the language

Developers are afraid to move their code from python 2 to python 3

Hundreds of really awesome packages aren't available for python 3 yet

2to3 - attempt to convert python 2 code to python 3 code

3to2 - attempt to convert python 3 code to python 2 code

The way of the future!


Main differences

Python 2 Python 3print statement print functionByte strings Unicode strings (at last)range returns a list range returns an iterator/ returns floor for ints Single division operatordistinction between int and long no distinction

These are generally good things

Many python 3 features already in python 2.6 and up

Can access new features in the __future__ module

1 >>> 7 / 52 13 >>> from __future__ import division4 >>> 7 / 55 1.3999999999999999


Phew

Next week we will look at some specific modulesnumpy, scipy, sympy, matplotlibPython is better than Microsoft excelThe iPython interpreterDistributed computing?

These slides: http://www.netsoc.tcd.ie/~stesh/python/2.pdf

Last weeks' slides:http://www.netsoc.tcd.ie/~stesh/python/1.pdf

Questions?




Documents

Python Tutorial #2