subins2000/answers.md

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    Module 6 Answers


Subin <subins2000@gmail.com>
21/11/2017
https://gist.github.com/subins2000/62db12a493f74d768b41934876f30aa5

August 2017


The following code sequence fails with a traceback when user enters a file that does not exist. How would you avoid the traceback and make it so you could print out your own error message when a bad file name was entered ?

fname = raw_input('Enter name: ')
fhand = open(fname)
Justify your answer.


Answer
A try-except block can be used to catch the error when the filename inputted by user doesn't exist :
#Python3
fname = input('Enter name: ')
try:
    fhand = open(fname)
except IoError:
    print('Unable to open file. Make sure the file exists')
When the file doesn't exist, calling open() will throw an exception IoError. This error will be caught in the program and a message will be printed.


Compare class and object. Generate a class to represent a rectangle.


Answer


class
object


Class is a blueprint or template from which objects are created
Object is an instance of class


Class is declared once
Object is created many times as per requirement


Class is defined with class keyword : 
class student: 
    name = ''
Object is created from class : 
 s1 = student()


This is a class to represent a rectangle :
class rectangle:
    length = 0
    breadth = 0


What is pickling ? How does it aid in putting values into a file? Also, what happens when the "load" method is invoked ?


Answer
Pickling is used for serializing and de-serializing a Python object structure. Any object in Python can be pickled so that it can be saved on disk and reused later. What pickle does is that it “serialises” the object first before writing it to file. Pickling is a way to convert a python object (list, dict, etc.) into a character stream.
#Python3
import pickle
fp = open('file.txt', 'r')
# load the object from the file into var b
b = pickle.load(fp)
When the load() function is called, the passed string is de-serialized and made into the original object. This object will be stored in the variable b in the above example.


b) Write a program that reads a file and writes out a new file with the lines in reversed order (7)


Answer
#Python3
file = open('a.txt', 'r')
out = open('out.txt', 'a')

contents = file.readlines()
contents = reversed(contents)

for s in contents:
    out.write(s)

file.close()
out.close()


24 a) Write a function that gets input from the user and handles the Value Error exception. Describe how exceptions are handled in python (8)
b) Create a class Student with attributes name and roll no. and a method dataprint() for displaying the same. Create two instances of the class and call the method for each instance.


Answer
a.
#Python3
def getInput():
    a = input('Enter an input: ')
    
    # Check if it only has non alphabets
    if a.isalpha() == False:
        raise ValueError('Only alphabets are allowed')
In Python, exceptions are handled with a try-except block :
try:
    # Call our function
    getInput()
    
    # Do something else
except ValueError:
    # Handle the error
In the try block, the code to run is added. We can mention the exception to be caught after the except keyword. When the code is ran and the exception occurs, the rest of the code in try block is halted and the code inside except block is ran.
Exception will contain more information about the error. It can be received by using as keyword mentioning a variable :
try:
    getInput()
except ValueError as error:
    # Handle error
    print(error) # Prints "Only alphabets are allowed"
Handling exceptions are extremely useful because the program can be prevented from stopping execution when an error occurs.


January 2017


Distinguish between object identity and structural equivalence.


Answer


Object Identity
Structural Equivalence


Two objects are considered to be the same object based on having identical properties
Two objects are structurally equivalent if they have the same physical instance


Objects are identical if they are instances of the same class
Objects are structural equivalent if class and all attributes of both are the same


a = myClass() 
 b = myClass() 
 b.property = 1 
 a is b # Returns True
a = myClass() 
 b = myClass() 
 b.property = 1 
 a.property == b.property # Returns False


List out the different modes in which a file can be opened in Python


Answer
w (Write Mode)

open('filename', 'w')
This mode is used to write to a file. The file will be created if it doesn't exist.
r (Read Mode)

open('filename', 'r')
This mode is used to read from a file. The file should exist. Otherwise an IoError is thrown.
a (Append Mode)

open('filename', 'a')
This mode is used to append contents to a file.


What do you mean by pickling in python? Explain its significance with the help of example.


Answer
Pickling is used for serializing and de-serializing a Python object structure. Any object in Python can be pickled so that it can be saved on disk and reused later. What pickle does is that it “serialises” the object first before writing it to file. Pickling is a way to convert a python object (list, dict, etc.) into a character stream.
#Python3
import pickle
fp = open('file.txt', 'w')
a = ['hello', 'world']
pickle.dump(a, fp)

fp = open('file.txt', 'r')
b = pickle.load(fp)

print(a == b) # Prints True


24 (a) Define the terms class,attribute,method and instance with the help of an example. (4)
(b) Create a class person with attributes Name, age, salary and a method display() for showing the details. Create two instances of the class and call the method for each instance. (5)
(c) Write a python program that opens a file for input and prints the count of four letter words in it. (5)


Answer
a)
 #Python3
 class student:
     name = ''

     def printHello(self):
         print('Hello')
class

Class is a data structure that combines multiple types of data into one. Here, student is the class.
attribute

Attributes are properties of a class to which values are stored. Here, name is an attribute
method

Methods are the functions inside the class. Here, printHello() is a method.
Instance

#Python3
s1 = student()
s1.name = 'Vyshak'
An object of a class is called an instance. Any changes to it will reflect only in that object
b)
#Python3
class person:
    name = ''
    age = 0
    salary = 0

    def display(self):
        print('Name: ', self.name)
        print('Age: ', self.age)
        print('Salary: ', self.salary)

p1 = person()
p1.name = 'Vishnu'
p1.age = 19
p1.salary = 40000
p2.name = 'Sudeep'
p2.age = 19
p2.salary = 40000

p1.printDetails()
p2.printDetails()
c)
#Python3
fp = open('file.txt', 'r')
s = fp.read()
count = 0

for word in s.split(' '):
    if len(word) == 4:
        count += 1

fp.close()

print('Number of 4 letter words: ', count)


June 2016


Why exception handling is required in programming


Answer
When runtime errors occur in a program, the whole program will be stopped. To prevent this, we can catch the errors and still continue the execution of the program.
The caught errors can be outputted to a log file for debugging. Thus the problem can be found out and the program will still run without stopping.


Differentiate Shallow equality and Deep equality


Answer
Shallow Equality = Object Identity
Deep Equality = Structural Equivalence
See answer of 12th question of January 2017 paper


Repeatation of quetion 16 of January 2017 paper
List advantage of using Pickling


Answer

Objects can be saved to and read from file
Objects can be reused later
User data can be conveniently saved


b) Repeatation of a previous question (try-except)


a) Write a Python code to read a text file, copy the contents to another file after removing the blank lines
b) Repeatation of previous question (class, attributes, instances)


Answer
a.
#Python3
inputFile = open('file.txt', 'r')
outFile = open('out.txt', 'w')

while True:
    line = inputFile.readline()

    if line == '':
        break

    # \n indicates a newline (linebreak). A blank line means that it has nothing but a linebreak
    if line != '\n':
        outFile.write(line)

inputFile.close()
outFile.close()


January 2016


Repeat of question January 2017 question 16


Repeat of question August 2017 question 24 a.


b) Write a Python program to create a text file and to input a line of text to it. Display the line of text with all punctuation marks removed.


Answer
#Python3
toWrite = input('Enter a text: ')
fp = open('file.txt', 'w')
fp.write(toWrite)
fp.close()

fp = open('file.txt', 'r')
contents = fp.read()
for l in contents:
    if l != '.' and l != '!' and l != '?'
        print(l, end='')

fp.close()


24 a) Repeat of question 24(a) of January 2017
c) Write a Python program to create a file containing 10 numbers. Read the contents of the file and display the square of each number.


Answer
c)
#Python3
fp = open('file.txt', 'w')
for i in range(0, 10):
    number = input('Enter a number: ')
    fp.write(number)
fp.close()

fp = open('file.txt', 'r')
while True:
    line = fp.readline()

    if line == '':
        break

    # Line will have a number only. So square it
    print(line * line)

fp.close()
class	object
Class is a blueprint or template from which objects are created	Object is an instance of class
Class is declared once	Object is created many times as per requirement
Class is defined with `class` keyword : class student: name = ''	Object is created from class : s1 = student()
Object Identity	Structural Equivalence
Two objects are considered to be the same object based on having identical properties	Two objects are structurally equivalent if they have the same physical instance
Objects are identical if they are instances of the same class	Objects are structural equivalent if class and all attributes of both are the same
a = myClass() b = myClass() b.property = 1 a is b # Returns True	a = myClass() b = myClass() b.property = 1 a.property == b.property # Returns False