[Python] Python Classes and Object Oriented Programming

[MrGrj]

Cel mai bun tutorial pe care l-am citit so far. Give it a try:

Spolier:

So Everything Has A Class?

Classes can be thought of as blueprints for creating objects. When I define a Customer class using the class keyword, I haven't actually created a customer. Instead, what I've created is a sort of instruction manual for constructing "customer" objects. Let's look at the following example code:

class Customer(object):
    """A customer of ABC Bank with a checking account. Customers have the
    following properties:

    Attributes:
        name: A string representing the customer's name.
        balance: A float tracking the current balance of the customer's account.
    """

    def __init__(self, name, balance=0.0):
        """Return a Customer object whose name is *name* and starting
        balance is *balance*."""
        self.name = name
        self.balance = balance

    def withdraw(self, amount):
        """Return the balance remaining after withdrawing *amount*
        dollars."""
        if amount > self.balance:
            raise RuntimeError('Amount greater than available balance.')
        self.balance -= amount
        return self.balance

    def deposit(self, amount):
        """Return the balance remaining after depositing *amount*
        dollars."""
        self.balance += amount
        return self.balance

The class Customer(object) line does not create a new customer. That is, just because we've defined a Customer doesn't mean we've created one; we've merely outlined the blueprint to create a Customer object. To do so, we call the class's __init__ method with the proper number of arguments (minus self, which we'll get to in a moment).

So, to use the "blueprint" that we created by defining the class Customer (which is used to create Customer objects), we call the class name almost as if it were a function: jeff = Customer('Jeff Knupp', 1000.0). This line simply says "use the Customer blueprint to create me a new object, which I'll refer to as jeff."

The jeff object, known as an instance, is the realized version of the Customer class. Before we called Customer(), no Customer object existed. We can, of course, create as many Customer objects as we'd like. There is still, however, only one Customer class, regardless of how many instances of the class we create.

self?

So what's with that self parameter to all of the Customer methods? What is it? Why, it's the instance, of course! Put another way, a method like withdraw defines the instructions for withdrawing money from some abstract customer's account. Calling jeff.withdraw(100.0) puts those instructions to use on the jeff instance.

So when we say def withdraw(self, amount):, we're saying, "here's how you withdraw money from a Customer object (which we'll call self) and a dollar figure (which we'll call amount). self is the instance of the Customer that withdraw is being called on. That's not me making analogies, either. jeff.withdraw(100.0) is just shorthand for Customer.withdraw(jeff, 100.0), which is perfectly valid (if not often seen) code.