Kurage Notes

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Objects ​

Attributes and Methods ​

Objects have attributes Methods are attributes bound to a function

python
>>> someObject.attribute
someReturnValue
>>> someObject.method(parameters)
someOtherReturnValue

Classes ​

A class serves as a template for all objects whose type is that class. Every object is an instance of some particular class. Defining a class uses a single clause

python
class <name>:
	<suite>

The suite is executed when the class statement is executed.

When a class is called:

  1. A new instance of that class is created.
  2. The __init__ method of the class is called with the new object as its first argument (named self), along with any additional arguments provided in the call expression.
python
class Account:
	def __init__(self, account_holder): # SELF is bounded to the object instance
		self.balance = 0 # Assigning attribute values
		self.holder = account_holder
a = Account('Jim')
  • __init__ is called a constructor
    • Whenever the class is called, __init__ is called with its arguments
    • This makes instance attributes
  • self is referred to a particular instance of the class Account
    • It's just a naming convention

Object Representation ​

See String Representation

Object Identity ​

Every object that is an instance of a user-defined class has a unique identity. Every call to a class creates a new class instance

Methods ​

Methods are defined in the suite of a class statement.

python
class Account:
	def __init__(self, account_holder): # SELF is bounded to the object instance
		self.balance = 0 # Assigning attribute values
		self.holder = account_holder
	def deposit(self, amount):
		self.balance += amount
		return self.balance
	def withdraw(self, amount):
		if amount > self.balance:
			return "Insufficient!"
		self.balance -= amount
		return self.balance

Invoking Methods ​

All invoked methods have access to the object via the self parameter, and so they can all access and manipulate the object's state.

Dot notation automatically supplies the first argument to a method. It supplies the object instance that we're invoking on.

python
>>> tomAccount = Account('Tom')
>>> tomAccount.deposit(100)
100

Dot Expressions ​

Object receive messages via dot notation. Dot notation access attributes of the instance or its class.

python
<expression>.<name>
  • The <expression> can be any valid Python expression.
  • The <name> must be a single name.

Evaluates to the value of the attribute looked up by <name> in the object that is the value of the <expression>

  • Is the <name> bound to the object?
  • If not, is the <name> bound to the class?

Methods and Functions ​

Bound methods couple together a function and the object on which that method will be invoked. The object will be passed in as the first argument of the method as a function

Object + Function = Bound Methods

python
>>> Account.deposit(tom_account, 1000)
1000
>>> tom_account.deposit(1000)
2000
>>> type(Account.deposit)
<class 'function'>
>>> type(tom_account.deposit)
<class 'method'>

These has the same effect.

Attributes ​

Attributes are data stored within either an instance or the class.

Accessing Attributes ​

  • getattr(name, 'attribute')
    • Equivalent to name.attribute
  • hasattr(name, 'attribute')
    • Return True if
      • The instance name has attribute attribute
      • The class has attribute attribute

Looking Up Attributes by Name ​

Evaluating a dot expression <expression>.<name>:

  1. Evaluate the <expression> to the left of the dot, which yields the object of the dot expression.
  2. <name> is matched against the instance attributes of that object; if an atttribute with that name exists, its value is returned.
  3. If not, <name> is looked up in the class, which yields a class attribute value.
  4. That value is returned unless it is a function, in which case a bound method is returned instead.

Class Attributes ​

Class attributes are "shared" across all instances of a class.

Class attributes can be looked up if the instance attribute is not found.

Assignment to Attributes ​

Assignment statements with a dot expression on their left-hand side affect attributes for the object of that dot expression

  • If the object is an instance, then assignment sets an instance attribute
  • If the object is a class, then assign sets a class attribute
python
Account.interest = 0.04 # Class Attribute
tom_account.interest = 0.06 # Instance Attribute