Mastering Python Logic: Template Method and Bridge Patterns

Prerequisites

To follow this tutorial, you should have a solid grasp of Python basics, including classes and inheritance. Familiarity with Object-Oriented Programming (OOP) and the concept of Abstract Base Classes is essential, as these serve as the backbone for both design patterns.

Key Libraries & Tools

• abc (Abstract Base Classes): A built-in Python module used to define blueprints for other classes, ensuring subclasses implement specific methods.
• Trading Bot Example: A practical context involving cryptocurrency exchanges like Binance and Coinbase to demonstrate real-world decoupling.

The Template Method Walkthrough

The Template Method defines the skeleton of an algorithm in a base class but lets subclasses override specific steps without changing the overall structure. We start by defining a TradingBot that outlines the check_prices process.

from abc import ABC, abstractmethod

class TradingBot(ABC):
    def check_prices(self, coin):
        self.exchange.connect()
        prices = self.exchange.get_market_data(coin)
        if self.should_buy(prices):
            print(f"Buying {coin}")
        if self.should_sell(prices):
            print(f"Selling {coin}")

    @abstractmethod
    def should_buy(self, prices):
        pass

    @abstractmethod
    def should_sell(self, prices):
        pass

By making should_buy and should_sell abstract, we force specific implementations like an AverageTrader or MinMaxTrader to define their own logic while keeping the execution flow identical.

Bridging Independent Variations

While the Template Method handles internal logic steps, the Bridge Pattern decouples an abstraction from its implementation so the two can vary independently. In our bot, the "Exchange" (where data comes from) is a separate concern from the "Trader" (how we decide to trade).

class Exchange(ABC):
    @abstractmethod
    def connect(self):
        pass

class BinanceExchange(Exchange):
    def connect(self):
        print("Connecting to Binance...")

We pass an Exchange instance into the TradingBot. This allows us to swap Binance for Coinbase without touching a single line of our trading strategy logic.

Tips & Gotchas

Always prioritize cohesion. A class should do one thing well. Avoid the temptation to use these patterns for very simple scripts where a basic function suffices; over-engineering adds unnecessary boilerplate. When debugging, remember that you cannot instantiate abstract classes directly—Python will throw an error if you forget to implement even one abstract method in your subclass.