spsl v unit - final

Shell Programming & Scripting Languages

Dr. K. Sasidhar

UNIT – VI Contents Object-Orientation

Data in Python, Data Structures in Python, Defining Classes

The Python Database Interface. Database Interfaces, The Underlying Interface Model,

Some Database Modules, A Simple Database-Driven Web, SQL/Python Communication.

Unit VI outcomes From the VI unit Student can

Implement user defined type class. Design oops concepts like inheritance. Understand, learn Python interface with databases. Connect with database and can write solution as a python

program and executes.

Object Oriented Programming in Python Class: A user-defined prototype for an object that defines

a set of attributes that characterize any object of the class. The attributes are data members (class variables and instance variables) and methods, accessed via dot notation.

Object: Instance of a class. An object comprises both data members (class variables and instance variables) and methods.

OOP in Python: Defining a class A class is a special data type which defines how to build a

certain kind of object. The class also stores some data items that are shared by all the

instances of this class Instances are objects that are created which follow the

definition given inside of the class Python doesn’t use separate class interface definitions as in

some languages You just define the class and then use it

Defining Methods Define a method in a class by including function definitions within the

scope of the class block There must be a special first argument self in all method definitions which

gets bound to the calling instance There is usually a special method called __init__ in most classes There is no “new” keyword in python Use the class name with ( ) notation and assign the result to a

variable __init__ serves as a constructor for the class. Usually does some

initialization work The arguments passed to the class name are given to its __init__()

method So, the __init__ method for employee is passed “Bob” and 15000

and the new class instance is bound to b: b=Employee(“Bob”,15000)

Simple class example class shareddata(object): a=44

x=shareddata() y=shareddata() z=shareddata()

print(x.a) print(y.a) print(z.a)

# Creates three instances

# Inherit and share a

Creating classes class ClassName:

class variables methods # methods are functions inside a classExample:

class Employee: empCount = 0 def __init__(self, name, salary): #__init__ runs when a new instance

object is created. #self is an

instance self.name = name self.salary = salary Employee.empCount += 1 def displayCount(self):

print(“count”, self.empCount) def displayDetails(self): print( "Name : ", self.name, ", Salary: ", self.salary )

Constructor: __init__

An __init__ method can take any number of arguments.

Like other functions or methods, the arguments can be defined with default values, making them optional to the caller.

However, the first argument self in the definition of __init__ is special…

Program to update the employee salary.

class Person: def __init__(self, name, job=None, pay=0): self.name = name self.job = job self.pay = pay def lastName(self): # Behavior methods return self.name.split()[-1] # self is implied

subject def giveRaise(self, percent): self.pay = int(self.pay * (1 + percent)) # Must change

here only if __name__ == '__main__': bob = Person(' Smith') sue = Person(' Jones', job='dev', pay=10000) print(bob.name, bob.pay) print(sue.name, sue.pay) print(bob.lastName(), sue.lastName()) # Use the new

methods sue.giveRaise(.10) # instead of hardcoding print(sue.pay)

Inheritance

Subclasses A class can extend the definition of another class

Allows use (or extension ) of methods and attributes already defined in the previous one.

To define a subclass, put the name of the superclass in parentheses after the subclass’s name on the first line of the definition. Class ECM_student(student):Python has no ‘extends’ keyword like Java.Multiple inheritance is supported.

Redefining Methods

To redefine a method of the parent class, include a new definition using the same name in the subclass. The old code won’t get executed.

To execute the method in the parent class in addition to new code for some method, explicitly call the parent’s version of the method.parentClass.methodName(self, a, b, c) The only time you ever explicitly pass ‘self’ as an argument is

when calling a method of an ancestor.

Definition of a class extending student

Class Student: #“A class representing a student.”def __init__(self,n,a): self.full_name = n self.age = a

def get_age(self): return self.age

Class ECM_student (student): #class extending student.”

def __init__(self,n,a,s): student.__init__(self,n,a) #Call __init__ for student self.section_num = s

def get_age(): #Redefines get_age method entirely print “Age: ” + str(self.age)

Database Programming The Python standard for database interfaces is the Python DB-

API. Python Database API supports a wide range of database servers

such as: MySQL PostgreSQL Microsoft SQL Server 2000 Informix Interbase Oracle Sybase GadFly mSQL

Working with Databases through Python Download a separate DB API module for each database

you need to access. For example, to access an Oracle database as well as a

MySQL database, download both the Oracle and the MySQL database modules.

Database connectivity and display version import MySQLdb # Open database connection db =

MySQLdb.connect("localhost","testuser","test123","TESTDB" )

# prepare a cursor object using cursor() method cursor = db.cursor() # execute SQL query using execute() method. cursor.execute("SELECT VERSION()") # Fetch a single row using fetchone() method. data = cursor.fetchone() print "Database version : %s " % data # disconnect from server db.close()

Creating a Table Using python import MySQLdb # Open database connection db =


# prepare a cursor object using cursor() method cursor = db.cursor() # Drop table if it already exist using execute() method. cursor.execute("DROP TABLE IF EXISTS EMPLOYEE") # Create table as per requirement sql = """CREATE TABLE EMPLOYEE ( empid Number(20) NOT NULL, NAME CHAR(20), Job char(10), Sal number(10)) cursor.execute(sql) db.close

READ Operation

READ Operation on any database means to fetch some useful information from the database.

Once database connection is established, then we can send a query into this database.

use either fetchone() method to fetch single record. fetchall() method to fetch multiple values from a database

table. fetchone(): It fetches the next row of a query result set.

A result set is an object that is returned when a cursor object is used to query a table.

fetchall(): It fetches all the rows in a result set. If some rows have already been extracted from the result set, then it retrieves the remaining rows from the result set.

rowcount: This is a read-only attribute and returns the number of rows that were affected by an execute() method.

Program to display all the records from EMPLOYEE table having salary more than 1000:

import MySQLdb db =


# prepare a cursor object using cursor() method cursor = db.cursor() # Prepare SQL query to INSERT a record into the database. sql = "SELECT * FROM EMPLOYEE \ WHERE INCOME > '%d'" % (1000) try: cursor.execute(sql)

results = cursor.fetchall() for row in results: fname = row[0] lname = row[1] age = row[2] sex = row[3] income = row[4] # print fetched result print "fname=%s,lname=%s,age=%d,sex=%s,income=

%d" % \ (fname, lname, age, sex, income ) except: print "Error: unable to fecth data" db.close()

Python Data Structures

Lists An ordered group of items Does not need to be the same type

Could put numbers, strings or donkeys in the same list

List notation A = [1,”This is a list”, c, Donkey(“kong”)]

Methods of Lists

List.append(x) adds an item to the end of the list

List.extend(L) Extend the list by appending all in the given list L

List.insert(I,x) Inserts an item at index I

List.remove(x) Removes the first item from the list whose value is

x

Examples of other methods a = [66.25, 333, 333, 1, 1234.5] //Defines List

print a.count(333), a.count(66.25), a.count('x') //calls method

2 1 0 //output a.index(333)

//Returns the first index where the given value appears

1 //ouput a.reverse() //Reverses order of list

a //Prints list a [333, 1234.5, 1, 333, -1, 66.25] //Ouput

a.sort() a //Prints list a [-1, 1, 66.25, 333, 333, 1234.5] //Output

Using Lists as Stacks The last element added is the first element retrieved To add an item to the stack,

append() must be used stack = [3, 4, 5] stack.append(6) Stack is now [3, 4, 5, 6]

To retrieve an item from the top of the stack, pop must be used Stack.pop() 6 is output Stack is now [3, 4, 5] again

Using Lists as Queues

First element added is the first element retrieved To do this collections.deque

must be implemented

List Programming Tools

Filter(function, sequence) Returns a sequence consisting of the items from the sequence

for which function(item) is true

Computes primes up to 25

Map Function Map(function, sequence)

Calls function(item) for each of the sequence’s items

Computes the cube for the range of 1 to 11

Reduce Function

Reduce(function, sequence) Returns a single value constructed by calling the binary

function (function)

Computes the sum of the numbers 1 to 10

The del statement A specific index or range can be deleted

Tuples Tuple

A number of values separated by commas Immutable

Cannot assign values to individual items of a tuple However tuples can contain mutable objects such as lists

Single items must be defined using a comma Singleton = ‘hello’,

Sets An unordered collection with no duplicate elements Basket = [‘apple’, ‘orange’, ‘apple’, ‘pear’] Fruit = set(basket) Fruit

Set([‘orange’, ‘apple’, ‘pear’])

Dictionaries Indexed by keys

This can be any immutable type (strings, numbers…) Tuples can be used if they contain only immutable

objects

Looping Techniques Iteritems():

for retrieving key and values through a dictionary

Looping Techniques Enumerate():

for the position index and values in a sequence

Zip(): for looping over two or more sequences

Comparisons Operators “in” and “not in” can be used to see

if an item exists in a sequence Comparisons can be chained

a < b == c This tests whether a is less than b and that b equals c

Fundamentals of Python: From First Programs Through Data Structures 39

Tree Terminology


Tree Terminology (continued)


Note: The height of a tree containing one node is 0By convention, the height of an empty tree is –1

Tree Terminology (continued)


General Trees and Binary Trees In a binary tree, each node has at most two

children: The left child and the right child


Recursive Definitions of Trees A general tree is either empty or consists of a finite set of

nodes T Node r is called the root The set T – {r} is partitioned into disjoint subsets, each of

which is a general tree A binary tree is either empty or consists of a root plus a left

subtree and a right subtree, each of which are binary trees


Why Use a Tree? A parse tree describes the syntactic

structure of a particular sentence in terms of its component parts


Why Use a Tree? (continued) File system structures are also tree-like


Why Use a Tree? (continued) Sorted collections can also be represented as

tree-like structures Called a binary search tree, or BST for short

Can support logarithmic searches and insertions


The Shape of Binary Trees The shape of a binary tree can be described

more formally by specifying the relationship between its height and the number of nodes contained in it

N nodesHeight: N – 1

A full binary tree contains the maximum number of nodes for a givenheight H


The Shape of Binary Trees (continued)

The number of nodes, N, contained in a full binary tree of height H is 2H + 1 – 1

The height, H, of a full binary tree with N nodes is log2(N + 1) – 1

The maximum amount of work that it takes to access a given node in a full binary tree is O(log N)


The Shape of Binary Trees (continued)


Three Common Applications of Binary Trees In this section, we introduce three special

uses of binary trees that impose an ordering on their data: Heaps Binary search trees Expression trees


A Binary Tree ADT Provides many common operations required

for building more specialized types of trees Should support basic operations for creating

trees, determining if a tree is empty, and traversing a tree

Remaining operations focus on accessing, replacing, or removing the component parts of a nonempty binary tree—its root, left subtree, and right subtree


The Interface for a Binary Tree ADT


The Interface for a Binary Tree ADT (continued)


Processing a Binary Tree

Many algorithms for processing binary trees follow the trees’ recursive structure

Programmers are occasionally interested in the frontier, or set of leaf nodes, of a tree Example: Frontier of parse tree for English sentence shown

earlier contains the words in the sentence


Processing a Binary Tree (continued)

frontier expects a binary tree and returns a list Two base cases:

Tree is empty return an empty list Tree is a leaf node return a list containing root item


Implementing a Binary Tree


Implementing a Binary Tree (continued)


The String Representation of a Tree

__str__ can be implemented with any of the traversals


Developing a Binary Search Tree A BST imposes a special ordering on the

nodes in a binary tree, so as to support logarithmic searches and insertions

In this section, we use the binary tree ADT to develop a binary search tree, and assess its performance


The Binary Search Tree Interface The interface for a BST should include a constructor and

basic methods to test a tree for emptiness, determine the number of items, add an item, remove an item, and search for an item

Another useful method is __iter__, which allows users to traverse the items in BST with a for loop


Data Structures for the Implementation of BST

…


Searching a Binary Search Tree find returns the first matching item if the target item is in

the tree; otherwise, it returns None We can use a recursive strategy


Inserting an Item into a Binary Search Tree add inserts an item in its proper place in the BST Item’s proper place will be in one of three positions:

The root node, if the tree is already empty A node in the current node’s left subtree, if new item is less

than item in current node A node in the current node’s right subtree, if new item is

greater than or equal to item in current node For options 2 and 3, add uses a recursive helper function

named addHelper In all cases, an item is added as a leaf node


Removing an Item from a Binary Search Tree Save a reference to root node Locate node to be removed, its parent, and its parent’s

reference to this node If item is not in tree, return None Otherwise, if node has a left and right child, replace

node’s value with largest value in left subtree and delete that value’s node from left subtree Otherwise, set parent’s reference to node to node’s only child

Reset root node to saved reference Decrement size and return item


Removing an Item from a Binary Search Tree (continued) Fourth step is fairly complex: Can be factored out into a

helper function, which takes node to be deleted as a parameter (node containing item to be removed is referred to as the top node): Search top node’s left subtree for node containing the largest

item (rightmost node of the subtree) Replace top node’s value with the item If top node’s left child contained the largest item, set top node’s

left child to its left child’s left child Otherwise, set parent node’s right child to that right child’s left

child


Complexity Analysis of Binary Search Trees BSTs are set up with intent of replicating O(log n)

behavior for the binary search of a sorted list A BST can also provide fast insertions Optimal behavior depends on height of tree

A perfectly balanced tree supports logarithmic searches Worst case (items are inserted in sorted order): tree’s height is

linear, as is its search behavior

Insertions in random order result in a tree with close-to-optimal search behavior


Case Study: Parsing and Expression Trees Request:

Write a program that uses an expression tree to evaluate expressions or convert them to alternative forms

Analysis: Like the parser developed in Chapter 17, current program

parses an input expression and prints syntax error messages if errors occur

If expression is syntactically correct, program prints its value and its prefix, infix, and postfix representations


Case Study: Parsing and Expression Trees (continued)



Design and Implementation of the Node Classes:



Design and Implementation of the Parser Class: Easiest to build an expression tree with a parser that uses a

recursive descent strategy parse should now return an expression tree to its caller,

which uses that tree to obtain information about the expression

factor processes either a number or an expression nested in parentheses Calls expression to parse nested expressions


An Array Implementation of Binary Trees An array-based implementation of a binary

tree is difficult to define and practical only in some cases

For complete binary trees, there is an elegant and efficient array-based representation Elements are stored by level

The array representation of a binary tree is pretty rare and is used mainly to implement a heap


An Array Implementation of Binary Trees (continued)


Implementing Heaps


Implementing Heaps (continued)

At most, log2n comparisons must be made to walk up the tree from the bottom, so add is O(log n)

Method may trigger a doubling in the array size O(n), but amortized over all additions, it is O(1)


Using a Heap to Implement a Priority Queue we can use a heap

spsl v unit - final

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