Using inheritance to provide one class by another in Python

Question

I'm trying to become a self taught developer, so I have nobody to ask when I do not understand the task in a course. The question is about inheritance. Given structure

I have to implement some classes as a piece of program to monitor news. I was given the base abstract class Trigger

class Trigger(object):
    def evaluate(self, story):
        """
        Returns True if an alert should be generated
        for the given news item, or False otherwise.
        """
        # DO NOT CHANGE THIS!
        raise NotImplementedError

And then I have to implement other classes accordingly the scheme. I did it like that:

class PhraseTrigger(Trigger):
    
    def __init__(self, trigger, story):
        self._trigger = trigger
        self._story = story
    
    def is_phrase_in(self):

        phrase_to_match = r"((\W)+|^)"
        for i in self._trigger.split():
            phrase_to_match += i
            if (i != self._trigger.split()[-1]):
                phrase_to_match += r"(\W)+"
            else:
                phrase_to_match += r"((\W)+|$)"

        # print(phrase_to_match) # delete after debugging
        scheme_to_match = re.compile(phrase_to_match, re.IGNORECASE)
        mo = scheme_to_match.search(self._story)

        if (mo != None):
            return True
        else:
            return False
        
    def evaluate(self, story):
        """
        Returns True if an alert should be generated
        for the given news item, or False otherwise.
        """
        
        if is_phrase_in(self):
            return True
        else:
            return False

I've done all of them except Not-, And- and OrTrigger. Next I was said "The NotTrigger 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". How can I add another class to init method of NotTrigger? Or I did not catch the idea completely. Any advice will be appreciated. P.S. Sorry for long text and my poor English(it's my second language).

Answer 1

Let's start with a simple implementation of NotTrigger. Here is how you would define an initializer (technically not the constructor) that accepts another trigger:

class NotTrigger:
    def __init__(self, trigger):
        self.trigger = trigger

Now the evaluate method should be trivial:

def evaluate(self, story):
    return not self.trigger.evaluate(story)

Hopefully you now see some of the problems with your implementation of PhraseTrigger:

1. PhraseTrigger should not accept neither another trigger as an argument to __init__, nor the story.
2. PhraseTrigger should not directly implement evaluate: that's up to the child classes.
3. PhraseTrigger should accept a phrase to search for.
4. PhraseTrigger should provide a method is_phrase_in(self, text) to search for self.phrase in text. Child classes will determine what to pass in for text.