10.2.2 Account Class Definition

Defining a Class

• Class definition begins with the keyword class followed by the class’s name and a colon (:)
• Called the class header
• Style Guide for Python Code recommends that you begin each word in a multi-word class name with an uppercase letter
• Every statement in a class’s suite is indented

# account.py
"""Account class definition."""
from decimal import Decimal

class Account:
    """Account class for maintaining a bank account balance."""

Defining a Class (cont.)

• Each class typically provides a descriptive docstring
• Must appear in the line or lines immediately following the class header
• View any class’s docstring in IPython, type the class name and a question mark, then press Enter

Account?

Init signature: Account(name, balance)
Docstring:      Account class for maintaining a bank account balance.
Init docstring: Initialize an Account object.
File:           ~/Dropbox/books/2019/PythonFullThrottle/account.py
Type:           type
Subclasses:     

Defining a Class (cont.)

• Account is both the class name and the name used in a constructor expression to create an Account object and invoke the class’s __init__ method
• IPython’s help mechanism shows both the class’s docstring ("Docstring:") and the __init__ method’s docstring ("Init docstring:")

Initializing Account Objects: Method __init__

• Constructor expression creates a new object, then initializes its data by calling the class’s __init__ method
• Each new class can provide an __init__ method that specifies how to initialize an object’s data attributes
• Returning a value other than None from __init__ results in a TypeError
• Class Account’s __init__ method initializes an Account object’s name and balance attributes if the balance is valid

Initializing Account Objects: Method __init__ (cont.)

def __init__(self, name, balance):
        """Initialize an Account object."""

        # if balance is less than 0.00, raise an exception
        if balance < Decimal('0.00'):
            raise ValueError('Initial balance must be >= to 0.00.')

        self.name = name
        self.balance = balance

Initializing Account Objects: Method __init__ (cont.)

• When you call a method for a specific object, Python implicitly passes a reference to that object as the method’s first argument
• So all methods of a class must specify at least one parameter
• By convention a method’s first parameter is named self
• Methods must use that reference (self) to access the object’s attributes and other methods

Initializing Account Objects: Method __init__ (cont.)

• When an object is created, it does not yet have any attributes
• They’re added dynamically via assignments of the form:

  self.attribute_name `=` value
  

Initializing Account Objects: Method __init__ (cont.)

• Python classes may define many special methods, like __init__
• Each identified by leading and trailing double-underscores (__) in the method name
• Class object defines the special methods that are available for all Python objects

Method deposit

• Adds a positive amount to the account’s balance attribute
• Raises a ValueError if amount is less than 0.00

def deposit(self, amount):
        """Deposit money to the account."""

        # if amount is less than 0.00, raise an exception
        if amount < Decimal('0.00'):
            raise ValueError('amount must be positive.')

        self.balance += amount

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