strategy.py

class BaseLogStrategy(object):
    def log(self, message):
        raise NotImplementedError()


class StdoutStrategy(BaseLogStrategy):
    def log(self, message):
        print(message)


class FileStrategy(BaseLogStrategy):
    def __init__(self, filename):
        self.filename = filename

    def log(self,message):
        with open(self.filename,'a') as f:
            f.write(message + '\n')


class App(object):
    def __init__(self):
        self.users = []
        self.log_strategy = None

    def set_strategy(self,log_strategy):
        self.log_strategy = log_strategy

    def add_user(self, user):
        self.users.append(user)
        self.log_strategy.log("Added user "+user)

    def exist_user(self, user):
        self.log_strategy.log("Finding user " + user)
        return user in self.users


app = App()
out_strategy = StdoutStrategy()
app.set_strategy(out_strategy)
app.add_user("Lol")
app.add_user("KEK")
file_strategy = FileStrategy("out.txt")
app.set_strategy(file_strategy)
app.add_user("Cheburek")
app.add_user("Arbidol")