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Free A-Level Computer Science guide
Learn how a class creates objects, how attributes hold state, how methods change it, and what Python's double underscores actually do.
Content owner: Michael Print · Written for A-Level learners · Checked against official specifications
Object-oriented programming (OOP) groups related state and behaviour in objects. A class defines a reusable type; each object is a separate instance of that class.
In the example below, each Score object has its own player and points attributes. Methods add points or return the current total, so other code does not need to change the stored points directly.
class Score: defines a reusable type of score record.
alex = Score("Alex") creates one object; sam is another.
self.__player and self.__points store each object's state.
addPoints() and getPoints() are functions defined on the class and called on an object.
class Score:
def __init__(self, player):
self.__player = player
self.__points = 0
def addPoints(self, amount):
if amount > 0:
self.__points += amount
def getPoints(self):
return self.__points
alex = Score("Alex")
sam = Score("Sam")
alex.addPoints(5)
print(alex.getPoints()) # 5
print(sam.getPoints()) # 0class Score:def __init__(self, player):Score("Alex") supplies the player name; Python supplies the new instance as self. You do not write Score(self, "Alex") when creating it.self.__points = 0Score object gets its own total.alex.addPoints(5)alex. Python supplies alex as the method's self, while 5 becomes amount.Worked example
alex and sam begin with zero points. Calling alex.addPoints(5) changes only alex's points attribute. The two print statements therefore output 5 and 0 on separate lines.
Encapsulation means keeping an object's state and the operations on it together, then exposing a deliberate way for other code to use that object. Here, callers use addPoints() and getPoints() instead of changing the stored total directly. The method can reject a zero or negative addition without changing the calling code.
The double underscores in __player and __points trigger Python name mangling. That discourages accidental access and name clashes, but it does not enforce strict privacy. These methods are an interface choice; a getter or setter is not required for every Python attribute.
These are original practice questions with indicative answers.
In the code above, identify the class, one object, one attribute and one method.
Indicative answer: Score is the class (1), alex is an instance (1), __points is an instance attribute (1), and getPoints is a method (1).
State the output of the two print statements and explain why the values differ.
Indicative answer: The output is 5 followed by 0 (1 mark each). alex.addPoints(5) changes only alex's points attribute (1); sam's points remain at the initial value of 0 (1).
What does the double underscore in self.__points do in Python? Explain one limitation of calling it “private”.
Indicative answer: The double-leading-underscore name is mangled by Python (1), which reduces accidental external use or subclass name clashes (1), but it is not a strict access barrier (1).
The linked-list page shows another application of classes and methods. It includes the complete workshop program and a captioned explanation.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
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Use the free examples first, then explore one-to-one tuition if guided practice would help.