Programming Fundamentals › Object-Oriented Programming

Constructor

The method that initializes a new object.

Also known as: constructors, __init__, initializer

A constructor is the method that runs when a new object is created, and sets up its starting state.

class User:
    def __init__(self, name, email):      # Python's constructor
        self.name = name
        self.email = email
        self.active = True

u = User("Ana", "ana@example.com")

class User {
  constructor(name, email) {
    this.name = name;
    this.email = email;
  }
}
const u = new User("Ana", "ana@example.com");

class User {
    User(String name, String email) { ... }    // same name as the class
}

What belongs in one

• Storing the arguments as fields.
• Setting default values.
• Checking that the arguments make sense, so an invalid object can’t exist (raise an error on a negative quantity, an empty name).

What doesn’t

• Slow or risky work: network calls, reading files, starting threads. A constructor that can fail in complicated ways is hard to test and use. Use a factory function (User.from_row(...)) or an explicit connect() step.
• Complicated logic. Keep it simple.

Related ideas

• Default arguments make constructors flexible (default parameters).
• Dependencies passed in instead of created inside make code testable (dependency injection).
• Subclasses call the parent constructor (super().__init__(...)) so the parent’s setup still runs (inheritance).
• Some languages allow several constructors (overloading), while Python and JavaScript have one, so people use default arguments or class methods.
• A forgotten new in JavaScript, or a forgotten self. in Python, are classic mistakes.