[SOLVED] Cmput 175 – lab 5: exceptions

Goal: Learn about inheritance in object-oriented programming, and practice handling
exceptions.
For a simple tutorial on inheritance in Python, see:
https://www.w3schools.com/python/python_inheritance.aspExercise 1:
Inheritance is a fundamental concept in object-oriented programming. It allows us to define a
new class that has all of the methods and properties of another class – just like a child inherits
traits from a parent, the new subclass inherits properties and behaviours from its base class. It is
also possible for the new subclass to have new properties and behaviours in addition to those it
inherited, or to override some of the inherited properties and/or behaviours with its own unique
ones.To create a subclass in Python, you create a new class definition as normal, and then add the
name of the base class (the one you want to inherit from) in brackets after the new subclass
name. For example, let’s define a Cake class and a BirthdayCake class, where the BirthdayCake
class inherits all properties and behaviours from the Cake class and also has a unique behaviour
of its own (putting birthday candles on top):
class Cake:
def __init__(self, flavour, frosted):
self.flavour = flavour
self.frosted = frosteddef bake(self, temp):
print(‘Cake is baking at’, temp, ‘degrees Celsius.’)
class BirthdayCake(Cake):
def putOnCandles(self, numCandles):
if numCandles > 0:
print(numCandles, ‘candles lit on cake; ready to be blown out.’)You can create objects that are instances of the BirthdayCake class, just the same way as you do
for any class in Python. For example:
grandmasCake = BirthdayCake(‘chocolate’, ‘True’)
grandmasCake.bake(180)
grandmasCake.putOnCandles(90)When an instance of BirthdayCake is created, the flavour and whether it is frosted or not must be
specified, just like for any cake. Also like any cake, it should be baked. But not all cakes have
candles put on it – just birthday cakes.Every type of exception that we use in Python is a subclass of the built-in Exception class. In
this exercise, you will make your own custom exception.
1. Download and save lab5_inheritance.py from eClass. Write a class definition to create a
new type of exception. The class name should be TransitionError.
a. This class should be a subclass of the Exception class.
b. This class should have its own __init__ method to override the init method (and its
attributes) that it inherited from the Exception class. This new method will have two parameters (besides the self parameter):
beginning and final.
 Inside the method, you should assign beginning and final to the new attributes
previousState and nextState, respectively.
 Inside the method, you will also create a third new attribute: msg, which is a string
explaining that we cannot transition from the previousState to the
nextState.c. This class should have a new method called printMsg(). It simply prints the value
of the msg attribute.2. Test your TransitionError class by running the main function already written for you in
lab5_inheritance.py. What is the difference between the second and third try statement?
i.e. why does it print different results?
Sample run:
Checking first person…Pebbles Flintstone started life as a(n) baby and ended as a(n) adult
This is the transition we expect as people grow old
Checking second person…
Benjamin Button started life as a(n) adult and ended as a(n) baby
Transition error: Not normal to transition from adult to baby
Checking second person again…
General error
Goodbye…Exercise 2:
You are tasked with writing a program that reads in information about client bank accounts
(where all of the clients, luckily, have unique names), and then allows the user to enter
transactions (withdrawals or deposits) for any of those accounts. You will need to catch and
handle specific exceptions, as described below.1. Create a new file, lab5_accounts.py. In this file, create a main() function that asks the user
for the name of the file to read the account data from. If the file does not exist, your main
function should handle the resulting OSError exception by printing an error message and
exiting the entire program gracefully (i.e. without displaying the call stack and exception
information).
Sample run when non-existing filename is entered:
Enter filename > sillyfile.txt
File error: sillyfile.txt does not exist
Exiting program…goodbye.If the file exists, main should open the file in read mode and then pass the resulting
TextIOWrapper object (i.e. the file handler) as an argument to the readAccounts function
(see step 2). The main function should then call the processAccounts function with the
appropriate input argument (see step 3).2. Create a function called readAccounts(infile) to read all of the information in the
account text file into a dictionary, one line at a time. Each line in the text file should contain a
(unique) name, then a greater than symbol (>), then a decimal number representing the
account’s opening balance. If the opening balance is not a number, this function should catch
the ValueError exception that results when you try to convert a non-number to a float. As
part of handling that exception, a warning message should be displayed (see sample run
below).After catching the exception, the function should continue on to the next line in the
file, without adding anything about the invalid account to the dictionary. Once all of the valid
account information has been added to the dictionary, this function should return the
dictionary.Sample run for invalid opening balance:
Input file example:
Bob>234.70
Mike Smith>Not_A_Float!
Warning! Account for Mike Smith not added: illegal value for balance3. Create a function called processAccounts(accounts), where the accounts parameter is a
dictionary. This function should ask the user to enter the name of a client, and the amount of
money to deposit or withdraw from that client’s account. After each successful transaction,
the amount of money left in the client’s account should be displayed. If the client name does
not exist as a key in the dictionary, the resulting KeyError that is raised when you try to
access the key should be handled by this function.For example, if Aladdin is entered by the
user, and he is not in the dictionary of clients, the following message would be displayed and
the user should be re-prompted to enter another client name:
Warning! Account for Aladdin does not exist. Transaction cancelled.Once a valid client name has been entered, the user is asked to enter a transaction amount
(positive for deposit, negative for withdrawal). This amount, if valid, will be added to the
client’s opening balance and displayed. You should update the value in the dictionary, but
you do not need to update the text file. If the user enters a value that cannot be converted to a
float number, the resulting ValueError is caught by this function so that the following
message is displayed:
Warning! Incorrect amount. Transaction cancelled.
After successfully handling the error, the function allows the user to continue entering client
names and transaction amounts until s/he enters Stop.4. Download and save a copy of accounts.txt from eClass. Test that all exceptions described
above are handled properly. Some of those exceptions (but not all) are shown in the sample
run below.
Sample Run for complete program:
Enter filename > accounts.txt
Warning! Account for Mike Smith not added: illegal value for balance
Warning! Account for George not added: illegal value for balance
Enter account name, or ‘Stop’ to exit: bob
Warning! Account for bob does not exist. Transaction cancelled
Enter account name, or ‘Stop’ to exit: Bob
Enter transaction amount for Bob: 30
New balance for account Bob: 264.7
Enter account name, or ‘Stop’ to exit: Zoya
Enter transaction amount for Zoya: hello
Warning! Incorrect amount. Transaction cancelled
Enter account name, or ‘Stop’ to exit: Anna
Enter transaction amount for Anna: -20.78
New balance for account Anna: 3239.77
Enter account name, or ‘Stop’ to exit: Bob
Enter transaction amount for Bob: 5.05
New balance for account Bob: 269.75
Enter account name, or ‘Stop’ to exit: Stop
Exiting program…goodbye.