Description
Introduction
In problem set 5, you will build a program to monitor news feeds over the Internet. Your
program will filter the news, alerting the user when it notices a news story that matches that
user’s interests (for example, the user may be interested in a notification whenever a story related
to the Red Sox is posted).
This problem set has a lot of words, but don’t get intimidated! The staff solution has about
80 lines of code; we recommend that the solutions you write for each problem should stay under
about 20 lines of code (the solutions for some problems will be much shorter than that). If you
find yourself writing way more code than that, you should come visit us at office hours to see
how you can simplify things.
We recommend starting early because there is a lot of reading here, but you ought to be able to
do this problem set sequentially in the order that we’ve laid out. There are a lot of references on
Python classes available (look for classes in the readings listed in the Readings & Reference
Section of the webpage); here is the official Python tutorial on classes, sections 9.1-9.7
(excepting 9.5.1) will be useful for this pset.
Getting Started
Download and save
1. You are provided with a zip file of all the files you need, including:
o ps5.py, a skeleton of a solution
o ps5_test.py, a test suite that will help you check your answers.
o triggers.txt, a sample trigger configuration file. You may modify this file to
try other trigger configurations.
o feedparser.py, a module that will retrieve and parse feeds for you.
o project_util.py, a module that includes a function to convert simple HTML
fragments to plain text.
o news_gui.py,a module that will pop up windows for you.
The three modules (feedparser.py, project_util.py, and news_gui.py) are
necessary for this lab to work, but you will not need to modify them. Feel free to read
through them if you’d like to understand what’s going on.
Contact the staff if you have trouble manipulating zip files.
RSS Overview
Many websites have content that is updated on an unpredictable schedule. News sites, such as
*RRJOHQHZV, are a good example of this. One tedious way to keep track of this changing
content is to load the website up in your browser,
and periodically hit the refresh button.
Fortunately, this process can be streamlined and automated by connecting to
the website’s RSS feed, using an RSS feed reader instead of a web browser
(e.g. Sage). An RSS reader will periodically collect and draw your attention
to updated content.
RSS stands for “Really Simple Syndication.” An RSS feed consists of
(periodically changing) data stored in an XML-format file residing on a web-server. For this
project the details are unimportant. You don’t need to know what XML is, nor do you need to
know how to access these files over the network.
We will use a special Python module to deal with these low-level details. The higher-level
details, in the notes below, describing the structure of the Google News RSS feed, should be
enough for our purposes.
Part I: Data structure design
RSS Feed Structure: Google News
First, let’s talk about one specific RSS feed: Google News. The URL for the Google News feed
is: http://news.google.com/?output=rss
If you try to load this URL in your browser, you’ll probably see your browser’s interpretation of
the XML code generated by the feed. You can view the XML source with your browser’s “View
Page Source” function, though it probably will not make much sense to you. Abstractly,
whenever you connect to the Google RSS feed, you receive a list of items. Each entry in this list
represents a single news item. In a Google News feed, every entry has the following fields:
• guid: A globally unique identifier for this news story.
• title: The news story’s headline.
• subject: A subject tag for this story (e.g. ‘Top Stories’, or ‘Sports’).
• summary: A paragraph or so summarizing the news story.
• link: A link to a web-site with the entire story.
Generalizing the Problem
This is, unfortunately, a little trickier than we’d like it to be, because each of these RSS feeds is
structured a little bit differently than the others. So, our goal in Part I is to come up with a
unified, standard representation that we’ll use to store a news story.
Why do we want this? When all is said and done, we want an application that aggregates several
RSS feeds from various sources, and can act on all of them in the exact same way: we should be
able to read the New York Times’s RSS feed, Google News’s RSS feed, The Tech’s RSS feed,
and the RSS feeds from blogs such as CuteOverload.com, all in one place.
Problem 1.
Parsing all of this information from the feeds that Google/Yahoo/the New York Times/etc. gives
us is no small feat. So, let’s tackle an easy part of the problem first: Pretend that someone has
already done the specific parsing, and has left you with variables that contain the following
information for a news story:
• globally unique identifier (GUID) – a string that serves as a unique name for this entry
• title – a string
• subject – a string
• summary – a string
• link to more content – a string
We want to store this information in an object that we can then pass around in the rest of our
program. Your task, in this problem, is to write a class, NewsStory, with at least the following
methods:
• get_guid()
• get_title()
• get_subject()
• get_summary()
• get_link()
You’ll also want to write a constructor for NewsStory that takes (guid, title, subject,
summary, link) as arguments and stores them appropriately. The solution to this problem
should be relatively short and very straightforward.
Parsing the Feed
Parsing is the process of turning a data stream into a structured format that is more convenient to
work with. We have provided you with code that will retrieve and parse the Google and Yahoo
news feeds.
Part II: Triggers
Given a set of news stories, your program will generate alerts for a subset of those stories.
Stories with alerts will be displayed to the user, and the other stories will be silently discarded.
We will represent alerting rules as triggers. A trigger is a rule that is evaluated over a single
news story and may fire to generate an alert. For example, a simple trigger could fire for every
news story whose title contained the word “Microsoft”. Another trigger may be set up to fire for
all news stories where the summary contained the word “Boston”. Finally, a more specific
trigger could be set up to fire only when a news story contained both the words “Microsoft” and
“Boston” in the summary.
In order to simplify our code, we will use object polymorphism. We will define a trigger
interface and then implement a number of different classes that implement that trigger interface
in different ways.
Trigger interface
Each trigger class you define should implement the following interface, either directly or
transitively. It must implement the evaluate method that takes a news item (NewsStory object)
as an input and returns True if an alert should be generated for that item. We will not use the
implementation of the Trigger class (which is why it throws an exception should anyone attempt
to use it), but rather the function definition that specifies that an evaluate(self, story) function
should exist.
The class below implements the Trigger interface (you will not modify this). Any subclass that
inherits from it will have an evaluate method. By default, they will use the evaluate method in
Trigger, the superclass, unless they define their own evaluate function, which would then be used
instead. If some subclass neglects to define its own evaluate() method, calls to it will go to
Trigger.evaluate(), which fails cleanly with the NotImplementedError exception:
class Trigger:
def evaluate(self, story):
“””
Returns True if an alert should be generated
for the given news item, or False otherwise.
“””
raise NotImplementedError
We will define a number of classes that inherit from Trigger. In the figure below, Trigger is a
superclass, which all other classes inherit from. The arrow from WordTrigger to Trigger means
that WordTrigger inherits from Trigger — a WordTrigger is a Trigger. Note that other classes
inherit from WordTrigger.
[Click on the above image for a full-size view]
Whole Word Triggers
Having a trigger that always fires isn’t interesting. Let’s write some that are. A user may want to
be alerted about news items that contain specific words. For instance, a simple trigger could fire
for every news item whose title contained the word “Microsoft”. In the following problems, we
ask you to create a word trigger abstract class and implement three classes that implement
triggers of this sort.
The trigger should fire when the whole word is present. For example, a trigger for “soft” should
fire on:
• Koala bears are soft and cuddly.
• I prefer pillows that are soft.
• Soft drinks are great.
• Soft’s the new pink!
• “Soft!” he exclaimed as he threw the football.
But should not fire on
• Microsoft announced today that pillows are bad.
This is a little tricky, especially the case with the apostrophe. For the purpose of your parsing,
pretend that a space or any character in string.punctuation is a word separator. If you’ve
never seen string.punctuation before, go to your interpreter and type:
>>> import string
>>> print string.punctuation
Play around with this a bit to get comfortable with what it is. The split and replace method of
strings will almost certainly be helpful as you tackle this part.
You may also find the string methods lower and/or upper useful for this problem.
Problem 2.
Implement a word trigger abstract class, WordTrigger. It should take in a string word as an
argument to the class’s constructor.
WordTrigger should be a subclass of Trigger. It has one new method, is_word_in.
is_word_in takes in one string argument text. It returns True if the whole word word is present
in text, False otherwise, as described in the above examples. This method should not be casesensitive. Implement this method.
Because this is an abstract class, we will not be directly instantiating any WordTriggers.
WordTrigger should inherit its evaluate method from Trigger. We do this because now we
can create subclasses of WordTrigger that use its is_word_in function. In this way, it is much
like the Trigger interface, except now actual code from the class is used.
Problem 3.
Implement a word trigger class, TitleTrigger that fires when a news item’s title contains a
given word. The word should be an argument to the class’s constructor. This trigger should not
be case-sensitive (it should treat “Intel” and “intel” as being equal).
For example, an instance of this type of trigger could be used to generate an alert whenever the
word “Intel” occurred in the title of a news item. Another instance could generate an alert
whenever the word “Microsoft” occurred in the title of an item.
Think carefully about what methods should be defined in TitleTrigger and what methods
should be inherited from the superclass.
Once you’ve implemented TitleTrigger, the TitleTrigger unit tests in our test suite should
pass.
Problem 4.
Implement a word trigger class, SubjectTrigger, that fires when a news item’s subject
contains a given word. The word should be an argument to the class’s constructor. This trigger
should not be case-sensitive.
Once you’ve implemented SubjectTrigger, the SubjectTrigger unit tests in our test suite
should pass.
Problem 5.
Implement a word trigger class, SummaryTrigger, that fires when a news item’s summary
contains a given word. The word should be an argument to the class’s constructor. This trigger
should not be case-sensitive.
Once you’ve implemented SummaryTrigger, the SummaryTrigger unit tests in our test suite
should pass.
Composite Triggers
So the triggers above are mildly interesting, but we want to do better: we want to ‘compose’ the
earlier triggers, to set up more powerful alert rules. For instance, we may want to raise an alert
only when both “google” and “stock” were present in the news item (an idea we can’t express
right now).
Note that these triggers are not word triggers and should not be subclasses of WordTrigger.
Problem 6.
Implement a NOT trigger (NotTrigger).
This trigger should produce its output by inverting the output of another trigger. The NOT trigger
should take this other trigger as an argument to its constructor (why its constructor? Because we
can’t change evaluate… that’d break our polymorphism). So, given a trigger T and a news item
x, the output of the NOT trigger’s evaluate method should be equivalent to not
T.evaluate(x).
When this is done, the NotTrigger unit tests should pass.
Problem 7.
Implement an AND trigger (AndTrigger).
This trigger should take two triggers as arguments to its constructor, and should fire on a news
story only if both of the inputted triggers would fire on that item.
When this is done, the AndTrigger unit tests should pass.
Problem 8.
Implement an OR trigger (OrTrigger).
This trigger should take two triggers as arguments to its constructor, and should fire if either one
(or both) of its inputted triggers would fire on that item.
When this is done, the OrTrigger unit tests should pass.
Phrase Triggers
At this point, you have no way of writing a trigger that matches on “New York City” — the only
triggers you know how to write would be a trigger that would fire on “New” AND “York” AND
“City” – which also fires on the phrase “New students at York University love the city”. It’s time
to fix this. Since here you’re asking for an exact match, we will require that the cases match, but
we’ll be a little more flexible on word matching. So, “New York City” will match:
• New York City sees movie premiere
• In the heart of New York City’s famous cafe
• New York Cityrandomtexttoproveapointhere
but will not match:
• I love new york city
Problem 9.
Implement a phrase trigger (PhraseTrigger) that fires when a given phrase is in any of the
subject, title, or summary. The phrase should be an argument to the class’s constructor. You may
find the Python operator in helpful, as in:
>>> print “New York City” in “In the heart of New York City’s famous cafe”
True
>>> print “New York City” in “I love new york city”
False
When this is done, the PhraseTrigger unit tests should pass.
Part III: Filtering
At this point, you can run ps5.py, and it will fetch and display Google and Yahoo news items
for you in little pop-up windows. How many news items? All of them.
Right now, the code we’ve given you in ps5.py gets all of the feeds every minute, and displays
the result. This is nice, but, remember, the goal here was to filter out only the the stories we
wanted.
Problem 10.
Write a function, filter_stories(stories, triggerlist) that takes in a list of news stories
and a list of triggers, and returns only the stories which a trigger fires for.
After completing Problem 10, you can try running ps5.py, and various RSS news items should
pop up, filtered by some hard-coded triggers defined for you in some code near the bottom. The
code runs an infinite loop, checking the RSS feed for new stories every 60 seconds.
Part IV: User-Specified Triggers
Right now, your triggers are specified in your Python code, and to change them, you have to edit
your program. This is very user-unfriendly. (Imagine if you had to edit the source code of your
web browser every time you wanted to add a bookmark!)
Instead, we want you to read your trigger configuration from a triggers.txt file, every time
your application starts, and use the triggers specified there.
Consider the following example configuration file:
# subject trigger named t1
t1 SUBJECT world
# title trigger named t2
t2 TITLE Intel
# phrase trigger named t3
t3 PHRASE New York City
# composite trigger named t4
t4 AND t2 t3
# the trigger set contains t1 and t4
ADD t1 t4
The example file specifies that four triggers should be created, and that two of those triggers
should be added to the trigger set:
• A trigger that fires when a subject contains the word ‘world’ (t1).
• A trigger that fires when the title contains the word ‘intel’ and
the news item contains the phrase ‘New York City’ somewhere (t4).
The two other triggers (t2 and t3) are created but not added to the trigger set directly. They are
used as arguments for the composite AND trigger’s definition.
Each line in this file does one of the following:
• is blank
• is a comment (begins with a
#)
• defines a named trigger
• adds triggers to the trigger
set.
Each type of line is described below.
Blank: blank lines are ignored. A line that consists only of whitespace is a blank line.
Comments: Any line that begins with a # character is ignored.
Trigger definitions: Lines that do not begin with the keyword ADD define named triggers. The
first element in a trigger definition is the name of the trigger. The name can be any combination
of letters without spaces, except for “ADD”. The second element of a
trigger definition is a keyword (e.g., TITLE, PHRASE, etc.) that specifies the kind of trigger being
defined. The remaining elements of the definition are the trigger arguments. What arguments are
required depends on the trigger type:
• TITLE: a single word.
• SUBJECT: a single word.
• SUMMARY: a single word.
• NOT: the name of the trigger that will be NOT’d.
• AND: the names of the two other triggers that will be AND’d.
• OR: the names of the two other triggers that will be OR’d.
• PHRASE: a phrase.
Trigger addition: A trigger definition should create a trigger and associate it with a name but
should not automatically add that trigger to the trigger set. One or more ADD lines in the .txt file
will specify which triggers should be in the trigger set. An addition line begins with the ADD
keyword. Following ADD are the names of one or more previously defined triggers. These triggers
will be added to the the trigger set.
Problem 11.
Finish implementing readTriggerConfig(filename). We’ve written code to open the file and
throw away all the lines that don’t begin with instructions (e.g. comments, blank spaces). Your
job is to finish the implementation. readTriggerConfig should return the list of triggers
specified in the configuration file.
Once that’s done, modify the code within the function main_thread to use the trigger list
specified in your configuration file, instead of the one we hard-coded for you:
# TODO: Problem 11
# After implementing readTriggerConfig, uncomment this line:
# triggerlist = readTriggerConfig(“triggers.txt”)
After completing Problem 11, you can try running ps5.py, and depending on your triggers.txt
file, various RSS news items should pop up for easy reading. The code runs an infinite loop,
checking the RSS feed for new stories every 60 seconds.
Hint: If no stories are popping up, open up triggers.txt and change the triggers to ones that
reflect current events (if you don’t keep up, just pick a trigger that would fire on one of the
current*RRJOHQHZV stories).
Handin Procedure
1. Save
All your code should be in a single file called ps5.py.
2. Time and collaboration info
At the start of the file, in a comment, write down the number of hours (roughly) you spent on this
problem set, and the names of whomever you collaborated with. For example:
# Problem Set 5
# Name: Jane Lee
# Collaborators (Discussion): John Doe
# Collaborators (Identical Solution): Jane Smith
# Time: 1:30
#
…. your code goes here …
3. Submit
You may upload new versions of the problem set until the 11:59pm deadline, but
anything uploaded after that time will be counted towards your late days, if you have any
remaining. If you have no remaining late days, you will receive zero credit for a late submission.
