{"id":3039,"date":"2021-11-24T02:34:59","date_gmt":"2021-11-24T02:34:59","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=3039"},"modified":"2025-03-31T10:01:56","modified_gmt":"2025-03-31T10:01:56","slug":"python-protocol","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-protocol\/","title":{"rendered":"Python Protocol"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the Python Protocol and its use to define implicit interfaces.<\/p>\n\n\n\n

Introduction to the Python Protocol #<\/a><\/h2>\n\n\n\n

Suppose you have a function<\/a> that calculates the total value of a product list, where each product has the name, quantity, and price attributes:<\/p>\n\n\n

from<\/span> typing import<\/span> List\n\n\nclass<\/span> Product<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name: str, quantity: float, price: float)<\/span>:<\/span>\n        self.name = name\n        self.quantity = quantity\n        self.price = price\n\n\ndef<\/span> calculate_total<\/span>(items: List[Product])<\/span> -> float:<\/span>\n    return<\/span> sum([item.quantity * item.price for<\/span> item in<\/span> items])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the calculate_total()<\/code> function accepts a list of Product<\/code> objects and returns the total value.<\/p>\n\n\n\n
When writing this function, you may want to calculate the total of a product list. But you likely want to use it for other lists such as inventory lists in the future.<\/p>\n\n\n\n
If you look closely at the calculate_total()<\/code> function, it only uses the quantity and price attributes.<\/p>\n\n\n\n
The following describes how to use the Protocol<\/code> class.<\/p>\n\n\n\n
First, define an Item<\/code> class that inherits from the Protocol<\/code> with two attributes: quantity and price:<\/p>\n\n\n
class<\/span> Item<\/span>(Protocol)<\/span>:<\/span>\n    quantity: float\n    price: float<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, change the calculate_total()<\/code> function that accepts a list of Item<\/code> objects instead of a list of Product<\/code> objects:<\/p>\n\n\n
def<\/span> calculate_total<\/span>(items: List[Item])<\/span> -> float:<\/span>\n    return<\/span> sum([item.quantity * item.price for<\/span> item in<\/span> items])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
By doing this, you can pass any list of Item<\/code> objects to the calculate_total()<\/code> function with the condition that each item has two attributes quantity and price.<\/p>\n\n\n\n
The following shows a complete program:<\/p>\n\n\n
from<\/span> typing import<\/span> List, Protocol\n\n\nclass<\/span> Item<\/span>(Protocol)<\/span>:<\/span>\n    quantity: float\n    price: float\n\n\nclass<\/span> Product<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name: str, quantity: float, price: float)<\/span>:<\/span>\n        self.name = name\n        self.quantity = quantity\n        self.price = price\n\n\ndef<\/span> calculate_total<\/span>(items: List[Item])<\/span> -> float:<\/span>\n    return<\/span> sum([item.quantity * item.price for<\/span> item in<\/span> items])\n\n\n# calculate total a product list<\/span>\ntotal = calculate_total([\n    Product('A'<\/span>, 10<\/span>, 150<\/span>),\n    Product('B'<\/span>, 5<\/span>, 250<\/span>)\n])\n\nprint(total)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
For example, you can define a list of stocks in inventory and pass them to the calculate_total()<\/code> function:<\/p>\n\n\n
\n# ...<\/span>\n\nclass<\/span> Stock<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, product_name, quantity, price)<\/span>:<\/span>\n        self.product_name = product_name\n        self.quantity = quantity\n        self.price = price\n\n\n# calculate total an inventory list<\/span>\ntotal = calculate_total([\n    Stock('Tablet'<\/span>, 5<\/span>, 950<\/span>),\n    Stock('Laptop'<\/span>, 10<\/span>, 850<\/span>)\n])\n\nprint(total)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the Product<\/code> and Stock<\/code> class don’t need to subclass the Item<\/code> class but still can be used in the calculate_total()<\/code> function.<\/p>\n\n\n\n
This is called duck typing in Python. In duck typing, the behaviors and properties of an object determine the object type, not the explicit type of the object.<\/p>\n\n\n\n
For example, an object with the quantity and price will follow the Item<\/code> protocol, regardless of its explicit type.<\/p>\n\n\n\n
The duck typing is inspired by the duck test: If it walks like a duck and its quacks like a duck, then it must be a duck.<\/strong><\/p>\n\n\n\n
<\/blockquote>\n\n\n\n
In practice, when you write a function that accepts input, you care more about the behaviors and properties of the input, not its explicit type.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
To make the calculate_total() more dynamic while leveraging type hints<\/a>, you can use the Protocol<\/code> from the typing<\/code> module. The Protocol class has been available since Python 3.8, described in PEP 544<\/a>.<\/p>\n\n\n\n