An Introduction to PythonApril 24th, 2012

Constanza F. SchibberWUStL

Wednesday, April 25, 2012

Why Python?• Productivity

• Reduced development time

• Code is extremely readable (I’ll show you today)

• Easy to learn

• Open-Source & Mature (over 20 years)

• Supportive community: Lots of resources available on-line!

• R through Python, or Python through R (it is much much faster than R)

• Object-oriented programming

• Fun!

• It is named after the BBC comedy series Monty Python’s Flying Circus

Wednesday, April 25, 2012

Popularity

http://blog.revolutionanalytics.comWednesday, April 25, 2012

Using Python• From a file (e.g. Command Line)

• Interactively

• Many softwares available.

• I use Eclipse.

• Available for Windows, Mac and Linux.

• Here are instructions for the configuration:

• http://www.youtube.com/watch?v=j4hido-FZKg

• http://www.vogella.com/articles/Python/article.html

• http://www.rose-hulman.edu/class/csse/resources/Eclipse/eclipse-python-configuration.htm Python is already installed

in Mac computers!!!

Wednesday, April 25, 2012

Today1. Python Types

2. Conditional Flow Statements: What if?

3. Iteration Statements: For Loop

4. Functions

5. Example 1: Saving webpages

6. Example 2: Word Frequency

7. What is NLTK?

Wednesday, April 25, 2012

Python Types

• Numbers

• Strings

• Lists and Tuples

• Dictionaries

Watch the Google Class!

Wednesday, April 25, 2012

Numbers• Natural numbers int

• Real number float

• Numeric operators 2+5 Summation

2*5 Multiplication

5**2 Exponentiation

5//2 Integer division

5%2 Division Remainder

5/2.0 Always be a real!

Wednesday, April 25, 2012

Number Comparison• bool

== , != equality/inequality

< , < = less than

>=, > greater than

bool(2=2)

bool(2!=3)

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Strings

• Text stored in data structure

• str

• Delimited by (“) or (‘)

hello=”Hello”

• Substrings

hello[0]

hello[1:4]

• Operators for Strings

Wednesday, April 25, 2012

Operators for Strings: Example

first= ‘John’

last= “Doe”

full=first+’ ‘ + last

print full

len(full)

Outputs John Doe, the length of full is 8 (counts space)

Wednesday, April 25, 2012

List• A list is a collection of one or more elements

• Lists are mutable

numbers=[1,2,3,4] sum/max/min

strings=[‘do’, ‘re’, ‘mi’]

lists=[[1,2],[3,4],[5,6]]

mylist=range(start, end, step)

fruits=[‘banana’, ‘kiwi’, ‘apple’, ‘mango’]

fruits.sort()

print fruits

Wednesday, April 25, 2012

Tuple

• A tuple is an object that bundles several related object with different types and semantics.

• What are the differences between tuple and list?

• tuples have their own methods

• tuples are immutable, lists are mutable

person=(“Doe”, [“John”, “J.”], 30)

Wednesday, April 25, 2012

Dictionary• A dictionary dict is an associative array where each

element is accessed by a name.

word_frequencies={

“good”:1,

“news”:2,

“everyone”:1,

“I”:3,

“have”:1}

word_frequencies[“news”]

‘news’ in word_frequencies

* Named element access with [ ]* Containment check with in

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What if?

if a != 0 :

print “a !=0”

else:

print “a == 0”

indentation is mandatory

Wednesday, April 25, 2012

for Loops• For each element in the container, execute a given suite of

elements.

fruits=[‘banana’, ‘kiwi’, ‘apple’, ‘mango’]

fruits.sort()

length =0 ## we want to know the total length of all the strings

for fruit in fruits:

length= length+len(fruit)

print length ## no indent is important here to get the total length!

Wednesday, April 25, 2012

Functions: Motivation

• Reusable code:

• We don’t want to write a complete algorithm every time.

• We can find functions written by other people and use them!

• Helps avoid problems with indentation when you have written a long code.

Wednesday, April 25, 2012

Example: Computing the Average

numbers= [2, 5, 8, 10]

type(numbers) # it is a list!!!

def avg_f(input):

s = sum(input)

l = len(input)

avg = s/l

return avg

avg_f(numbers)

indentation

function definition

output{Wednesday, April 25, 2012

Digression: Using Eclipse

• Eclipse tells you when you have an error:

• It gives you the error ‘before’ running the code.

• It specifies the line.

• It may give you some information about it.

• ALWAYS save as you are writing the code. You need to save before running the code. If next to the name of the module you see * , you have not saved your work.

• If you want to see results in between the code, just use print object. Very useful!

Wednesday, April 25, 2012

Example 1: Web

• Take advantage of the HTML code.

• It is useful in for loops.

• It is useful to locate words: You can identify words in italics or bold, titles, etc.

• It is useful to locate sections of the text.

• Take a look a Beautiful Soup module.

Wednesday, April 25, 2012

Example II: Word Count

• There are built in functions for this

• Look at NLTK

• Installing NLTK can be challenging. Look for a youtube video on how to do it. It is requires some special steps.

Wednesday, April 25, 2012

NLTK• NLTK is the acronym for Natural Language Toolkit

• A collection of python modules and objects tailored for NLP subtaks

Taks Modules Functionality

Accessing corpora nltk.corpusStandardized interfaces to corpora and lexicons

String processing nltk.tokenize nltk.stem Tokenizers, sentence tokenizers, stemmers

POS tagging nltk.tag Regular expressions, n-gram

Parsing nltk.parsechart, feature-baed, unification, probabilistic,

dependency

Semantic interpretation nltk.sem nltk.interference model checking, lambda calculus

... ... ...

Wednesday, April 25, 2012

(Some) Resources

www.python.org

wiki.python.org

Google Class (video)http://code.google.com/edu/languages/google-python-class/

Post questions or read answers to questionshttp://stackoverflow.com/

Wednesday, April 25, 2012