MIT-6.0001 Problem Set 5 Solution

Introduction
Here is the this pset. Chapter 8 in the textbook is also useful. official Python tutorial on classes, sections 9.1-9.7 (except for 9.5.1) will be useful for
Getting Started
Download and save
pset5.zip: A zip file of all the files you need, including:
● ps5.py, a skeleton for you to fill in
● ps5_test.py, a test suite that will help you check your answers
● triggers.txt, a trigger configuration file
● feedparser.py, a module that will retrieve and parse feeds for you


feed, using an RSS feed reader instead of a web browser. An RSS reader (e.g. Sage) 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 problem set, 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 have taken care of retrieving and parsing the XML file for you.
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 News 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.
● description : A paragraph or so summarizing the news story.
● link : A link to a website with the entire story.
● category : News category, such as “Top Stories”
Generalizing the Problem
This is 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 is to come up with a unified, standard representation that we'll use to store a news story.
We want to do this because 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 news stories from various RSS feeds all in one place.
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Problem 1
Parsing (see below for a definition) all of this information from the feeds that
Google/Yahoo/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
● title - a string
● description - a string
● link to more content - a string
● pubdate - a datetime
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 , starting with a constructor that takes (guid, title, description, link, pubdate ) as arguments and stores them appropriately. NewsStory also needs to contain the following methods:
● get_guid(self)
● get_title(self)
● get_description(self)
● get_link(self)
● get_pubdate(self)
The solution to this problem should be relatively short and very straightforward (please review what get methods should do if you find yourself writing multiple lines of code for each). Once you have implemented NewsStory all the NewsStory test cases should work.
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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.
Triggers
Given a set of news stories, your program will generate alerts for a subset of those stories.
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 directly
use the implementation of the Trigger class, which is why it raises an exception should anyone attempt to use it.
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 (albeit cleanly) with the NotImplementedError :
class Trigger(object):
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, from which all other classes inherit. The arrow from PhraseTrigger to Trigger means that PhraseTrigger inherits from Trigger - a PhraseTrigger is a Trigger . Note that other classes inherit from PhraseTrigger.

Phrase Triggers
assume that a phrase does not contain any punctuation. Here are some examples of valid phrases:
● 'purple cow'
● 'PURPLE COW'
● 'mOoOoOoO'
● 'this is a phrase'
But these are NOT valid phrases:
● 'purple cow???' (contains punctuation)
● 'purple cow' (contains multiple spaces between words)
Given some text, the trigger should fire only when each word in the phrase is present in its entirety and appears consecutively in the text, separated only by spaces or punctuation. The trigger should not be case sensitive. For example, a phrase trigger with the phrase "purple cow" should fire on the following text snippets:
● 'PURPLE COW'
● 'The purple cow is soft and cuddly.'
● 'The farmer owns a really PURPLE cow.'
● 'Purple!!! Cow!!!'
● 'purple@#$%cow'
● 'Did you see a purple cow?'
But it should not fire on these text snippets:
● 'Purple cows are cool!'
● 'The purple blob over there is a cow.'
● 'How now brown cow.'
● 'Cow!!! Purple!!!'
● 'purplecowpurplecowpurplecow'
>>> import string
>>> print string.punctuation
Play around with this a bit to get comfortable with what it is. The split, replace, join, methods of strings will almost certainly be helpful as you tackle this part.
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Problem 2
Implement a phrase trigger abstract class, PhraseTrigger. It should take in a string phrase as an argument to the class's constructor. This trigger should not be case-sensitive (it should treat "Intel" and "intel" as being equal).
PhraseTrigger should be a subclass of Trigger. It has one new method, is_phrase_in, which takes in one string argument text. It returns True if the whole phrase phrase is present in text, False otherwise, as described in the above examples. This method should not be case-sensitive. Implement this method.
Because this is an abstract class, we will not be directly instantiating any PhraseTriggers . PhraseTrigger should inherit its evaluate method from Trigger . We do this because now we can create subclasses of PhraseTrigger that use its is_phrase_in function.
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You are now ready to implement PhraseTrigger's two subclasses: TitleTrigger and DescriptionTrigger .
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Problem 3
Implement a phrase trigger subclass, TitleTrigger that fires when a news item's title contains a given phrase. For example, an instance of this type of trigger could be used to generate an alert whenever the phrase "Intel processors" occurred in the title of a news item.
As it was in PhaseTrigger, the phrase should be an argument to the class's constructor, and the trigger should not be case-sensitive.
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. Remember that all subclasses that inherit from the Trigger interface should include a working evaluate method.
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Problem 4
Implement a phrase trigger subclass, DescriptionTrigger , that fires when a news item's description contains a given phrase. As it was in PhaseTrigger, the phrase should be an argument to the class's constructor, and the trigger should not be case-sensitive.
Once you've implemented DescriptionTrigger , the DescriptionTrigger unit tests in our test suite should pass.
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Time Triggers
Let’s move on from PhraseTrigger. Now we want to have triggers that is based on when the NewsStory was published, not on its news content. Please check the earlier diagram if you’re confused about the inheritance structure of the Triggers in this problem set.
Problem 5
Because this is an abstract class, we will not be directly instantiating any TimeTrigger.
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Problem 6
Implement BeforeTrigger and AfterTrigger, two subclasses of TimeTrigger. BeforeTrigger fires when a story is published strictly before the trigger’s time, and AfterTrigger fires when a story is published strictly after the trigger’s time. Their evaluate should not take more than a couple of lines of code.
Once you've implemented BeforeTrigger and AfterTrigger, the BeforeAndAfterTrigger unit tests in our test suite should pass.
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Composite Triggers
Note that these triggers are not phrase triggers and should not be subclasses of
PhraseTrigger. Again, please refer back to the earlier diagram if you’re confused about the inheritance structure of Trigger.
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Problem 7
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 what parameters evaluate takes in...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.
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Problem 8
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.
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Problem 9
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.
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Filtering
At this point, you can run ps5.py, and it will fetch and display Google and Yahoo news items for in a pop-up window. How many news items? All of them.
Right now, the code we've given you in ps5.py gets the news from both 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.
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Problem 10
Write a function, filter_stories(stories, triggerlis t) that takes in a list of news stories and a list of triggers, and returns a list of only the stories for which a trigger fires.
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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:
// description trigger named t1 t1,DESCRIPTION,Presidential Election
// title trigger named t2 t2,TITLE,Hillary Clinton
// description trigger named t3 t3,DESCRIPTION,Donald Trump
// composite trigger named t4 t4,AND,t2,t3
// the trigger list 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 list:
● A trigger that fires when the description contains the phrase 'Presidential Election' ( t1).
● A trigger that fires when the title contains the description Donald Trump and the title contains the phrase Hillary Clinton (t4).
The two other triggers (t2 and t3) are created but not added to the trigger list directly. They are used as arguments for the composite AND trigger's definition (t4).
Each line in this file does one of the following:
● is blank
● is a comment (begins with // with no spaces preceding the //)
● defines a named trigger
● adds triggers to the trigger list 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 // is ignored.
Trigger definitions: Lines that do not begin with the keyword ADD define named triggers. Elements in these lines are separated by commas. The first element in a trigger definition is either the keyword ADD or the name of the trigger. The name can be any combination of letters/numbers, but it cannot be the word "ADD". The second element of a trigger definition is a keyword (e.g., TITLE, AND, etc.) that specifies the type of trigger being defined. The remaining elements of the definition are the trigger arguments. What arguments are required depends on the trigger type:
● TITLE: one phrase
● DESCRIPTION: one phrase
● AFTER: one correctly formatted time string
● BEFORE: one correctly formatted time string
● NOT: the name of the trigger that will be NOT'd ● AND: the names of the two triggers that will be AND'd. ● OR: the names of the two triggers that will be OR'd.
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 list. One or more ADD lines in the trigger configuration file will specify which triggers should be in the trigger list. An ADD line begins with the ADD keyword. The elements following ADD are the names of one or more previously defined triggers. The elements in these lines are also separated by commas. These triggers will be added to the the trigger list.
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Problem 11
Finish implementing read_trigger_config(filename) . We've written code to open the file and throw away all blank lines and comments. Your job is to finish the implementation. read_trigger_config should return a list of triggers specified by the configuration file.
Hint: Feel free to define a helper function if you wish! Using a helper function is not required though.
Hint: You will probably find it helpful to use a dictionary where the keys are trigger names.
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 read_trigger_config, uncomment this line:
# triggerlist = read_trigger_config('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. The code runs an infinite loop, checking the RSS feed for new stories every 120 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 with the news, just pick some triggers that would fire on the current Google News stories).
Problem 12
debate_triggers.txt so that you can get news that is published within the +/- 3 hour window of the debate!
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This completes the problem set!

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6.0001 Introduction to Computer Science and Programming in Python
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