OOP Files and Array

Programs rarely keep their data typed into the code. Instead the data lives in a text file, and the program loads it when it runs. In OOP that means: read each record from the file, build one object from it, and collect the objects in an array so you can search and process them.

This is one of the most common Paper 4 long-answer shapes. By the end of this page you will read records into objects, store them in an array with a counter, search the array, and wrap the file work in exception handling so a missing file does not crash the program.

Start with a class for one record. Here each animal in a zoo has a name, species and age, stored as private attributes with a display method.

class Animal:
    def __init__(self, name, species, age):
        self.__name    = name      # private
        self.__species = species   # private
        self.__age     = age       # private

    def get_name(self):
        return self.__name

    def display_info(self):
        print("Name:", self.__name, "Species:", self.__species, "Age:", self.__age)

If the file stores each animal on three consecutive lines (name, species, age), read three lines, build one object, and append it to a list. strip() removes the trailing newline; int() converts the age.

zoo = []                       # list to hold Animal objects
file = open("Animals.txt", "r")
for i in range(3):             # three animals in the file
    name    = file.readline().strip()
    species = file.readline().strip()
    age     = int(file.readline().strip())
    zoo.append(Animal(name, species, age))
file.close()

for animal in zoo:
    animal.display_info()

# Animals.txt:
# Leo
# Lion
# 5
# Bella
# Elephant
# 10
# ...

Declare the array with a capacity, initialise every slot to None, and keep an integer total of how many objects are stored. Store each new object at index total, then add 1 — but stop if the array is full.

MAX = 30
book_list = [None] * MAX     # global array of up to 30 Book objects
total = 0

def initialise_array():
    global total
    for i in range(MAX):
        book_list[i] = None
    total = 0

A linear search loops through the stored objects, comparing a getter's value to the target. Wrap the file open/read in try / except so a FileNotFoundError is caught.

def find_book(title_to_find):
    for i in range(total):
        if book_list[i].get_title() == title_to_find:
            return i        # found: return the index
    return -1               # not found

def load_books():
    try:
        file = open("Books.txt", "r")
        line = file.readline()
        while line != "":
            title, status = line.strip().split(",")
            store_book(Book(title, status))
            line = file.readline()
        file.close()
    except:
        print("File could not be opened.")

Fifteen exam-style tasks. Click Hint for bullet-point guidance, then Help to reveal a worked Python solution.