lesson 130 Classes & Objects · advanced

Class attributes and class methods

Some values belong to the class, not to each object: a VAT rate, a set of allowed statuses, the next invoice number. A class method works with the class itself, for example to build objects from a row of data.

You'll be able to

Define class attributes and read them through the class or through an object
Explain why self.counter += 1 creates an instance attribute instead of changing the class's
Write a @classmethod such as from_row that builds an object with cls(...)

The idea

A name assigned directly in the class body, outside any method, is a class attribute: one value shared by every object of the class. Constants such as a VAT rate fit here, and so does a counter that numbers invoices across all of them.

Reading self.VAT_RATE finds it: Python looks on the object first and, not finding it there, on the class. Writing is different: self.x = … always creates or changes an attribute on that one object. To change the shared value, assign through the class: Invoice.next_number += 1.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · Invoices numbered across the class

class Invoice:
    VAT_RATE = 0.2
    next_number = 1

    def __init__(self, customer_id, net):
        self.number = f"INV-{Invoice.next_number:04d}"
        Invoice.next_number += 1
        self.customer_id = customer_id
        self.net = net

    def gross(self):
        return round(self.net * (1 + self.VAT_RATE), 2)

first = Invoice("C00077", 100.0)
second = Invoice("C01338", 19.99)
print(first.number, first.gross())
print(second.number, second.gross())
print(Invoice.VAT_RATE, first.VAT_RATE, Invoice.next_number)

example-2.py · Counting through self

class Ticket:
    next_number = 1

    def __init__(self, subject):
        self.number = self.next_number
        self.next_number += 1
        self.subject = subject

first = Ticket("Parcel damaged")
second = Ticket("Wrong size")
print(first.number, second.number)
assert second.number == 2, f"both tickets got number {second.number}"

A class method receives the class instead of an object. Mark it with @classmethod on the line above def; its first parameter is called cls. Its usual job is an alternative constructor: a second way to build an object, from a CSV row or a line of text, that does the parsing and then calls cls(...) like the ordinary constructor. Callers write Product.from_row(row).

example-3.py · Products from a row or a line

class Product:
    def __init__(self, sku, name, price):
        self.sku = sku
        self.name = name
        self.price = price

    @classmethod
    def from_row(cls, row):
        return cls(row["sku"].strip().upper(), row["name"].strip(), float(row["price"]))

    @classmethod
    def from_line(cls, line):
        sku, name, price = line.split(";")
        return cls(sku.strip().upper(), name.strip(), float(price))

a = Product.from_row({"sku": " spo-11681", "name": "Yoga mat", "price": "31.00"})
b = Product.from_line("hom-22847; Desk lamp ;48")
print(a.sku, a.name, a.price)
print(b.sku, b.name, b.price)
print(type(b).__name__)

When to use it: class attributes for constants and values shared by every object; class methods for constructors that start from another format.

Mistakes to avoid: changing a shared counter through self (above); a list or dictionary as a class attribute that objects then append to (every object shares it; make per-object lists in __init__); and writing Product(...) inside a class method instead of cls(...).

Review

Class attributes are shared by every object; read them through self or the class.
Assigning through self creates an instance attribute; change shared values through the class.
@classmethod methods receive cls; alternative constructors return cls(...).
Next module: classes that build on other classes, and objects that contain objects.

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