Object-Oriented Programming
Home Lesson 4 — Containment or Inheritance?

Q3 is the prerequisite check. A data type can be another class. When you see a class name in the data type column of a design, that is the signal this whole lesson is built on. If you wrote "ARRAY" or "STRING", reread the row before you go on.

886
1119

• The first total is taken when the shelf holds two books (412 + 474). The second is taken after a third is added. The shelf remembers its books between calls because they are stored in its own attribute.
• A Shelf is not a kind of Book. A shelf has books: its attribute __Books is an array whose elements are Book objects. That relationship is called containment.
• The loop runs to self.__Count because that is how many books are actually stored. The array has three elements, but until a book is added an element holds None — there is no book there to ask.

Two traps: giving Screen3D its own copies of ScreenNumber and Seats (inheritance already provides them), and making Cinema inherit from Screen because the description mentions both in one sentence.

    def AddBook(self, NewBook):
        if self.__Count < 3:
            self.__Books[self.__Count] = NewBook
            self.__Count = self.__Count + 1
            return True
        else:
            return False

Fiction = Shelf("Fiction")
for NewBook in [Book("Dune", 412), Book("Emma", 474), Book("Holes", 233), Book("Beloved", 324)]:
    Added = Fiction.AddBook(NewBook)
    if Added == False:
        print(NewBook.GetTitle() + " did not fit")
print("Total pages: " + str(Fiction.TotalPages()))

Expected output:

Beloved did not fit
Total pages: 1119

• Check, then store, then count. The book goes into element self.__Count — the next free one — and only then does the counter go up. Doing it the other way round leaves element 0 empty and runs off the end one book early.
• A Boolean on every path. Both branches return something. A method that returns True when it works and nothing at all when it does not hands the caller None.
• Four separate lines of Added = Fiction.AddBook(...) and a test after each is equally good. The loop over a list of new books is just shorter.

It runs. It still loses most of the declaration marks.

• ✗ Inherits from Song. A playlist is not a song. Song is in the data type column of an attribute, so this is containment. The class-header mark is lost.
• ✗ Wrong parameters. The design gives the constructor one parameter, PlaylistName. Title and Artist belong to a song, and are only here because the candidate chose inheritance.
• ✗ No Songs array. The attribute that holds the songs is missing entirely. Every later part — adding a song, listing the songs, counting them — now has nowhere to work, so the error costs marks well beyond this part.
• ✓ PlaylistName and SongCount are private, typed, and SongCount is set to the fixed value 0 inside the constructor. Credited.

What would have scored

class Playlist:
    def __init__(self, PlaylistName):
        self.__PlaylistName = PlaylistName    # STRING
        self.__Songs = [None] * 50            # ARRAY[0:49] OF Song
        self.__SongCount = 0                  # INTEGER

The habit to build: before you write the class header, run your finger down the data type column. If the name of another class is there, it is containment — no brackets after the class name, no super(), and that class's parameters stay out of your constructor.

Copy this into your editor and build Club underneath it.

class Member:
    def __init__(self, MemberID, Name, Age):
        self.__MemberID = MemberID   # STRING
        self.__Name = Name           # STRING
        self.__Age = Age             # INTEGER
        self.__Credits = 0           # INTEGER
        self.__Active = True         # BOOLEAN

    def DisplayDetails(self):
        print("Member ID : " + self.__MemberID)
        print("Name      : " + self.__Name)
        print("Age       : " + str(self.__Age))
        print("Credits   : " + str(self.__Credits))
        print("Active    : " + str(self.__Active))

    def GetName(self):
        return self.__Name

    def GetCredits(self):
        return self.__Credits

    def SetAge(self, NewAge):
        self.__Age = NewAge

    def AddCredits(self, Amount):
        NewTotal = self.__Credits + Amount
        if NewTotal > 50:
            NewTotal = 50
        self.__Credits = NewTotal

Accepted alternatives

• MP5: testing for the full case first — if self.__NumberOfMembers == 5: return False — then storing.
• MP11–MP12: a Found flag tested after the loop, or a return straight after DisplayDetails() with the not-found output after the loop.
• MP13–MP14: six separate blocks of code, or a loop over a list of the test data — as long as the order is kept and AddMember's result is stored before it is tested.

Not credited, however good the code looks

• class Club(Member): — MP1 lost, and MP2 with it if the constructor takes a member's parameters.
• AddMember(self, MemberID, Name, Age) that builds the member inside the method — MP5 lost. The design passes a Member object.
• No full check — MP5 lost, and the program crashes on Tom Reilly, so MP16 goes too.
• TotalCredits() or DisplayMember() written as a function outside the class — no marks for that part. The design lists them as methods of Club.
• Looping range(0, 5) over the array instead of up to NumberOfMembers — MP8 lost. It happens to work here because the club is full; on any club with fewer than five members it crashes on None.
• self.__Members[Index].__Credits — MP9 lost. It crashes: the attribute is private to Member.
• The not-found message in an else inside the loop — MP12 lost.
• Calling AddMember without keeping what it returns, or calling it a second time inside the if — MP14 lost.
• Code pasted as an image — no code marks at all.

Extension — not counted in the 16

• E1 Captain is a private attribute with its type evidenced as Member, created in the constructor (None is accepted as "no captain").
• E2 SetCaptain() takes one parameter and assigns it to Captain — attribute on the left.
• E3 GetCaptainName() returns self.__Captain.GetName() — it asks the contained object, rather than reaching into it.
• E4 Omar Khalid's object is passed to SetCaptain(), and the output reads Captain: Omar Khalid.