What Is in a Name Modules and Classes

1
What Is in a Name?(Modules and Classes)
2
Modules

• Modules create a separate name space for
  functions and classes to be declared it.
• For instance,
• import sys
• sys.path.append(myModulePath)
• We have been using modules since the beginning

3
Modules

• Modules are just python files
• E.g. mymodule.py
• Modules are used with the import statement
• import mymodule
• Of course the interpreter needs to know where to
  find the specified

4
Oh, Module! Where are thou?

• Module location is system dependent information
• In the module sys there is a list called path
• This is the module search path
• It is a list of strings where each string is a
  directory path
• sys.path.append(myModulePath)

5
Why Would I Want to Use a Module?

• To group together related functions and variables
• All programs are modules
• To debug projects with the interactive debugger

6
Modules are Nice, But Do They Have Class?

• Someone may have noticed that in C classes
  allow you to group together functions and
  variable
• However, modules are not classes
• Python has class, too
• But they are a little bit different

7
ClassesWhat are They Good For?

• Creating new types
• What is a type?
• A set of variables (Class Variables) grouped with
  a set of function (Class Methods)
• Variable can be assigned type instances (e.g.
  class instances)
• You can think of a class as a module that you can
  instantiate a class instance and assign that
  instance to a variable

8
But What Does That Mean?

• It means that we can declare a class like Account
  to represent a bank account
• Lets say the each Account class has a function
  withdraw(Amount)
• Now we can use Accounts like any other type
• e Account("Bill", 50.0)
• e.withdraw(45.0)

9
But How Do We Declare a Class?

• It all starts with a declaraction
• class Account(object)
• A simple bank account record
• def __init__ (self, name, balance)
• Account Constructor
• self.__name name Create a private
  class variable name
• self.__balance float(balance) Create
  a private balance
• return

10
How Do I Make Some Real Methods?

• class Account(object)
• A simple bank account record
• def deposit(self, Amount)
• deposit Amount into self
• self.__balance Amount
• return
• def withdraw(self, Amount)
• withdraws Amount from self
• self.__balance - Amount
• return

11
What was all that?

• Class starts the class declaration
• For this class consider the use of object
  mandatory magic
• Functions declared in the class block are methods
  in the class
• __init__() is the name of the class constructor
• __init__() can have any number of parameters
• Names inside a class block that are prefixed __
  are private names

12
With Class Methods the First Parameter is Special

• Python does not have keyword like this or me to
  designate the calling class instance
• Calls to class methods pass the calling instance
  as the first parameter to the method function
• e.toString()
• Account.toString(e)
• Convention is to call the first parameter self

13
How Do I Make Some Real Methods?

• class Account(object)
• A simple bank account record
• def transfer(self, ToAccount, Amount)
• Transfers Amount from self to ToAccount
• self.withdraw(Amount)
• ToAccount.deposit(Amount)
• return
• def balance(self)
• return self.__balance
• def name(self)
• return self.__name

14
How Do I Make Some Real Methods?

• class Account(object)
• A simple bank account record
• def toString(self)
• return self.__name " 4.2f"
  self.__balance

15
Class Challenge

• Make a class called Bank that defines the
  following methods
• withdraw(Who, Amount)
• deposit(Who, Amount)
• transfer(Who, Amount)
• balance(Who)
• add(Who, Amount)
• toString()