CIT 590Intro to Programming

Style

Classes

Remember to finish up findAllCISCourses.py

Elements of style• Do not make an extremely long function. Break it up in

some manner • Generally in this course the long function will be your main

method• Remember coke-machine.py• Is there repeated code – just make a function out of it

• Do not write an incredibly long single line of code• In Python you can break a line up by using parantheses for

instance

• docStrings! – write a docString for every function. Triply quoted string telling the user what the function does

Elements of style• Break the program into small portions and write the

smallest portion first• Your functions need to be reusable

• Take pride in your functions. • Make your worst enemy want to use them• General purpose• Try and make them robust to bad inputs

• Write the smallest function first = write unit tests for the smallest function first

• The main function should be the last thing you complete

Elements of testing• Write more than 1 test case for each function• Check the test files that we used for your homeworks• Your tests and meant to find bugs so you do not want to

be gentle!• Adding tests is easy. Do not be hesitant about adding

more.• See movieTests.py in the repo• All unit tests being used for evaluation can be found in the

files section of canvas (we will organize it better)

Design Buzzwords• Don’t Repeat Yourself• As a software developer your goal always always is to

avoid copy pasting code.• The opposite of DRY is ….• WET – Write Everything Twice • Keep It Simple Stupid (KISS)• You Ain’t Gonna Need It (YAGNI)

• Reduce the number of components if you feel you are implementing functionality that you were not asked for

• 80-20 rule gets applied a lot in the industry

Classes• Encapsulate functions that can be used for a common

purpose• bankAccount.py• Classes implicitly apply the principles of abstraction and

information hiding• The user of the bank account class does not need to know how

balance is being maintained• Just get a handle to the class and use the deposit and withdrawal

functions

Class definition

class BankAccount(object): def __init__ (self, initBalance): self.balance = initBalance def deposit(self, amount): self.balance += amount

Parent class

methods

Creating an instance of the class• The class definition by itself does not create an instance

of the class• myFirstAccount = BankAccount()

• For those of you with some java background, there is no ‘new’

• An instance of a class is called an object

Methods• A class method looks very very much like a regular

function• The first argument HAS to be ‘self’• Calling methods within other methods

• We have to always refer to which object we are calling the method on

def transfer (self, amount, toAccount):

self.withdraw(amount)

toAccount.deposit(amount)

Constructors• The __init__ is implicitly called when you say

B = BankAccount()• First time we are seeing the __<function name>__

convention• Python uses this when we have functions that are used

behind the scenes

Objects are references => be careful

• Very similar to the situation of two lists• The copy module is your best friend

• import copy• bankAccount2 = copy.copy(bankAccount1)

• Passing an object as a function parameter• Again similar to lists, the object will be modified • Sometimes you want the modification• Sometimes you want to copy

How to print out an object• __str__

When you call print <objectname> this internal function will be called. Python will always provide some kind of default

However the default is ‘lame’

Try commenting out the __str__ in the bankAccount object

Accessing the data fields• Python classes expose their data fields. • For the bankaccount example you could always

manipulate the balance data field directly BUT this is considered bad programming practice

• It is considered much better form to provide what is called a getter and a setter• In this bank account example, a getter for balance is fully justified

since that is a service you would want• However a direct setting of bank balance = xyz seems

inappropriate. • We have deposit and withdraw

• More on this when we get to Java

Inheritance

• You’ve already seen it without it being explicitly called as such• Idea = Using methods defined on the parent class• Software reuse

• By inheriting from a class you only need to define the specialized methods• Often you might be inheriting from a different developer’s class

• Checking account example

class CheckingAccount(bankAccount.BankAccount)• Checking account inherits from bankaccount• Checking account is a special type of bankaccount

• When should I use inheritance• Apply the ‘is-a’ method• A real number is a complex number• A checking account is a bank account

Question on classes

Using one class inside another class

A bank contains a lot of bankAccounts.

bank = Bank()

What do we get when we do print Bank()

a) The account is owned by CEO balance is 100

b) The account is owned by CEO balance is 0

c) The account is owned by Arvind balance is 100

d) The account is owned by Arvind balance is 0

e) I am confused

Inbuilt functions to help explore inheritance

• Use isinstance• Issubclass• Using the types module (import types)• You can check for an int by doing

• Isinstance(3, types.IntType)

The is-a versus has-a concept• A common method used to identify inheritance is to ask

the question ‘Is class B a special case of class A?’ If the answer is yes, B extends A.

• If the answer is no, then you might want one class contained inside the other – composition

• A Bank contains BankAccounts - composition• A CheckingAccount is a BankAccount - inheritance