Programming Fundamentals › Object-Oriented Programming

Field / Property

A piece of data stored on an object.

Also known as: field, property, member variable, instance variable

An attribute (also called a field or property) is a piece of data stored on an object.

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

p = Product("Kettle", 29.99)
p.name                            # "Kettle"
p.price = 24.99                   # change it
p.stock = 12                      # Python lets you add new ones on the fly

const p = { name: "Kettle", price: 29.99 };    // object literal
p.name;

Each object has its own copy of its attributes, so two Product objects can hold different prices.

Kinds

• Instance attributes: belong to one object (self.price).
• Class attributes: shared by every object of the class (Product.tax_rate). Be careful with mutable ones such as lists, because all instances share them.
• Computed properties: look like attributes, but are calculated when read (getters and setters).

@property
def price_with_tax(self):
    return self.price * 1.2

Naming

Many languages use different words for similar things: field (Java, C#), property (JavaScript, C#), attribute (Python), member variable (C++). They all mean data on an object.

Habits

• Initialize every attribute in the constructor, so objects start complete (constructors).
• Keep internal ones private by convention or modifier (access modifiers).
• Don’t store what you can compute (a total from line items), or it can go out of sync.
• Misspelled names create silent bugs in dynamic languages: p.pric = 5 adds a new attribute.