Python Lab 11 (Objects and Classes)

Classes allow us to define entirely new types. We might think of each type in Python as a set of values, and we use a class declaration to describe how to build and interact with all the values of this brand new type. We will often need at minimum a constructor (telling us what instance variables to always create), str and repr methods (telling Python how to represent the thing as a string), and then any extra methods that we deem useful ways of interacting with the new values.


We can actually make our own Exception types by making a class that extends any existing Exception type. We'll make one in this lab and raise it a few times. If we made a larger project, we'd probably have some main menu that interacted with the user and would have to figure out how to recover from these raised exception values at some point. This lab has enough going on already, so we aren't doing this today.


Task 1 - Grade

A Grade represents a specific score on a specific kind of assessment (and has its own assumedly unique name). This is just a "container" type, used to give memorable names to individual grouped sub-values.


class Grade: Define the Grade class.

def __init__(self,  kind,  name,  percent):  Grade constructor. All three parameters must be stored to instance variables of the same names (kind, name, and percent). If kind is not one of "test", "lab", "project", or "final" (example: kind=="survey"), then raise a GradingError with the message "no  Grade  kind  'survey'". (You can skip this exception-raising part until later).

kind :: str. Should be something like "lab", "test", or "project".

name :: str. Could be something like "L1", "T2", or "P6".

percent :: int. (I chose int to simplify the assignment, but float would have been useful).

def __str__(self): returns a human-centric string representation. If kind=="test", name=="T1", and percent==97, then the returned string must be "test:T1(97%)" (no padded zeroes).

def __repr__(self): returns a computer-centric string representation. As is often the case, we want to construct a string that could be pasted back into the interactive mode to re-create this object. Given the same example as in __str__, we need to return the string "Grade('test', 'T1', 97)". Note: we choose to always use single-quotes.

def __eq__(self, other): we want to check that two grades are equal (our self and this other grade). We compare each instance variable like so (this is the definition!) return self.kind==other.kind and and self.percent==other.percent


Task 2 - GradeBook

This represents an entire grouping of Grade values as a list (named grades). We can then dig through this list for interesting things and calculations by calling the methods we're going to implement.


class GradeBook: Define the GradeBook class.

def __init__(self): GradeBook constructor. Create the only instance variable, a list named grades, and initialize it to an empty list. This means that we can only create an empty GradeBook and then add items to it later on.

def __str__(self): returns a human-centric string representation. We'll choose a multi-line representation (slightly unusual) that contains "GradeBook:" on the first line, and then each successive line is a tab, the str() representation of the next Grade in self.grades, and then a newline each time. This means that the last character of the string is guaranteed to be a newline (regardless of if we have zero or many Grade values).

def __repr__(self): We will be lazy and tell this to just represent the exact same thing as __str__. But don't cut-paste the code! Use this as the entire body of this function: return str(self)

def add_grade(self, grade): append the argument Grade to the end of self.grades

def average_by_kind(self, kind): Look through all stored  Grade objects  in  self.grades.  All those that are the same kind as the kind parameter should be averaged together (sum their percents and divide by the number of things of that kind). If none of that kind exist, return None.

def get_all_of(self, kind): create and return a list of references to each Grade object in this GradeBook that is of that kind.

def get_by_name(self, name): search through self.grades in order and return the first Grade by the given name. If no such Grade value can be found (say, name=="whatever"), then raise a GradingError with the message "no  Grade  found  named  'whatever'". (You can skip this exception-raising part and come back to it).


Task 3 – GradingError

The GradingError class extends the notion of an Exception (note the (Exception) part of the class signature line). We're making our own exception that stores a single string message.


class GradingError(Exception): Define the GradingError class to be a child of Exception class (by putting Exception in the parentheses as shown).

def __init__(self,msg): Constructor. Store msg to an instance variable named msg.

def __str__(self): human-centric representation of GradingError is just self.msg

def __repr__(self): computer-centric representation (that could be pasted back to interactive mode to recreate the object). We'll always use triple single quotes as part of the represented string when including the message. For instance, if self.msg=="bad grade", this method would need to return "GradingError('''bad grade''')"


If you skipped the two exception-raising parts above, you should now go back and add those checks/raises. The test cases for GradingError will go back and check those other parts correctly did so.


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