Lecture 2: Writing Your Own Class Definition

Transcription

1 Lecture 2: Writing Your Own Class Definition CS6507 Python Programming and Data Science Applications Dr Kieran T. Herley Department of Computer Science University College Cork

2 Summary Writing class definitions. Instance variable and functions. Initializing functions. Class variables and functions. 1

3 Modules in Python

4 Python modules Module Sources file containing Python code defs for functions and other code objects facilitates modularity and code re-use built in modules third party modules bespoke (write your own!) 2

5 Simple example Suppose file mymaths.py contains pi = hidden = def area(r ): Return area of circle with radius ' r '. global pi return pi r r Can access module objects from within other Python files By convention: internal objects not intended to be accessed outside module have names that begin with underscore 3

6 Simple example cont d Can access mymath stuff using import Approach 1: import mymaths print (mymaths.pi) Refer to things using modulename.thingname format Approach 2: from mymaths import pi print (pi) Refer to things just as thingname Approach 3: from mymaths import print (pi) 4

7 Python module path Executing import, Python searches a list of directories names Python module path the find the module Can see this math using IDLE (see File-Path Browser) or import sys print( sys. path) Can modify path in accommodate new modules; careful with this! 5

8 Class definition basics

9 Classes and objects Class Collection of code objects sharing common behaviour Behaviour specified by class definition ( blueprint ) Object Individual member of a class; program may employ many Implementations Built-in classes Third-party classes Bespoke classes 6

10 A bank-account class Imagine we need a bank-account class to capture details of individual for some financial application Data Operations Customer s name (assumed unique) Account balance (non-negative) Deposit some amount Withdraw some amount Check balance Could write application using these without detailed knowledge of internal Python details; all we need is a summary of behaviour i.e. services provided by bank-account objects 7

11 Class behaviour Each BankAccount object has the following behaviour. deposit(amount) withdraw(amount) get balance() Increase this account s balance by the value of amount. If sufficient balance in this account, reduce balance by value amount or to zero otherwise. Return the value of the reduction. Return the balance of this account. Note each individual BankAccount object has its own individual balance. 8

12 Working with BankAccount2 Can use make use of BankAccount2 without any understanding of its internal implementation detail Example: from bank account2 import acc1 = BankAccount2( Murphy, Tom, 100) # Create an account object acc1. deposit(250) acc1.withdraw(75) dosh = acc1.get balance() # Apply operations to object Note: Syntax for object creation and object function application similar to that for class date 9

13 Class definition Part 1 Class definition for implement bank account. class BankAccount2: Class to represent a simple bank account. # Code for constructor function named init # Code for functions deposit, withdraw and get balance 10

14 Visibility of instance variables Each object maintains two variables: balance the customer s balance name the customer s balance With class def. code these are referred to as self. balance Double underscores emphasize these are not intended to be accessible outside the module; users of BankAccout2 objects manipulate balance only via class functions deposit etc. 11

15 Class definition Part 2 def init ( self, nme =, bal = 0): Create a new account for account holder ' name'. self. name = nme self. balance = bal Special function called once when object is created Initializes any object data (here variables name balance and Note constructor (and other functions take) self as first parameter. 12

16 Class definition Part 2 def deposit( self, amount): Increase this account's balance by the value of 'amount'. self. balance = self. balance + amount def withdraw( self, amount): If sufficient balance in this account, reduce balance by value 'amount' or to zero otherwise. Return the value of the reduction. temp = min(self. balance, amount) self. balance = self. balance temp return temp def get balance( self ): Return the balance of this account. return self. balance 13

17 Notational quirk Function definition indicates two parameters: self and amount def deposit( self, amount): self. balance = self. balance + amount Function usage supplies only one argument (for amount) myaccount.deposit(1000) All functions in class definition take self as their first parameter 14

18 Class vs Instance Functions and Variables

19 Class variables and instance variables Instance variables Class variables Each object is an instance of its class Each instance embodies personal copy of instance variables e.g. balance for our bank-account example Class may also have class variables common to all objects within class i.e. one var.. for many instances e.g. what if we wanted to keep track of num. of bank accounts? 15

20 class BankAccount3: all accounts = [] def init ( self, nme =, bal = 0): self. name = nme self. balance = bal... Instance variable Class variable balance one per instance (account object) Code refer to it using self. balance all accounts only one for entire class (intended to hold the various instances) NB no self... 16

21 A picture BankAccount3: all accounts: BA1 BA2 BA3 BA1 name: Tom balance: deposit(...) withdraw(...) get balance() BA2 name: Dick balance: deposit(...) withdraw(...) get balance() BA3 name: Harry balance: deposit(...) withdraw(...) get balance() (Per-)Class function: num accounts() 17

22 Class Functions and Instance Functions def deposit( self, amount): self. balance = self. balance + amount def num accounts(): return len(bankaccount3.all accounts) Instance functions (Per-)Class functions One copy per instance Have self parameter Called as acc.deposit(250) Only one per class No self parameter Called as BankAccount3.sum accounts() 18

23 A closer look all accounts = [] def init ( self, nme =, bal = 0): self. name = nme self. balance = bal BankAccount3.all accounts.append( self)... def num accounts(): return len(bankaccount3.all accounts) 19

24 While we re at it Class functions to show accounts in decreasing order of balance Notes: def display accounts (): accounts = sorted(bankaccount3.all accounts,\ key = lambda x : x. balance, reverse = True) for acc in accounts: print(acc) sorted function takes two optional arguments reverse to select increasing or decreasing order key to select sorting key; note this can be a function 20

25 Technical Note Python supports two closely-related type of per-class functions: static functions and class functions. The differences are too arcane for us to concern us, so we will use the former (i.e. Python s static function). I use the term class function or per-class function I mean a function associated with the class as a whole even though strictly speaking this should be referred to as static function. 21

26 Back Material

27 Notes and Acknowledgements Reading Code Acknowledgements 22