oop - Call Python Method on Class Attribute Change

Question

I'm writing an API parsing Twitter bot and am very new to OOP. I have some existing Python code that relies on global variables and figured I could take this opportunity to learn.

I have the following Team class that gets updated when the API is parsed and is like to be able to call a totally unrelated (external) method when a class attribute changes.

class Team(object):
  def __init__(self, team_name, tri_code, goals, shots, goalie_pulled):
    self.team_name = team_name
    self.tri_code = tri_code
    self.goals = goals
    self.shots = shots
    self.goalie_pulled = goalie_pulled

When goalie_pulled is changed for an existing instance of Team I'd like the following method to be called (pseudo code):

def goalie_pulled_tweet(team):
  tweet = "{} has pulled their goalie with {} remaining!".format(team.team_name, game.period_remain)
  send_tweet(tweet)

Two things -

1. How do I call goalie_pulled_tweet from within my Team class once I detect that goalie_pulled attribute has changed?
2. Can I access an instance of my Game object from anywhere or does it need to be passed to that variable as well?

Answer 1

You should take a look at the property class. Basically, it lets you encapsulate behaviour and private members without the consumer even noticing it.

In your example, you may have a goalie_pulled property:

class Team(object):
    def __init__(self, team_name, tri_code, goals, shots, goalie_pulled):
        # Notice the identation here. This is very important.
        self.team_name = team_name
        self.tri_code = tri_code
        self.goals = goals
        self.shots = shots

        # Prefix your field with an underscore, this is Python standard way for defining private members
        self._goalie_pulled = goalie_pulled

    @property
    def goalie_pulled(self):
        return self._goalie_pulled

    @goalie_pulled.setter
    def goalie_pulled(self, new_value):
        self._goalie_pulled = new_value
        goalie_pulled_tweet(self) #self is the current Team instance

From the consumer's point of view:

team = create_team_instance()

# goalie_pulled_tweet is called
team.goalie_pulled = 'some_value'

I'd recommend you to use properties whenever you can (and must), as they are a nice way of abstraction.