← VCE Software Development U4O1

Protected attributes and methods

A single underscore prefix marks a protected member: by convention it is for use inside the class and its subclasses only. Python does not enforce it, so outside code can still access it.

Part of: VCE Software Development - U4O1 - development and evaluation — Module 77

Understanding Protected attributes and methods in VCE Software Development U4O1

What it does: The Savings subclass reads the parent's _balance attribute inside its own method, which is the intended use of a protected member. Python does not block this access and no error occurs, but it breaks the convention because the code is outside the class and its subclasses. The underscore tells other programmers that _name is for the class and its subclasses, not for general use. One leading underscore means protected. Two leading underscores make the member private and trigger name mangling.

Common mistakes: Dog must inherit from Pet to use the protected attribute _name. It should be class Dog(Pet): so subclass code can access self._name. A class header must end with a colon. It should be class Savings(Account): so the subclass can use the protected _balance.

Class: _____________________ · Name: _____________________ · Date: _________

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This module's key terms are also in a printable crossword (Modules 3–79) and word search (Modules 3–114).

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Learning

Duo

Which attribute name marks a protected member in Python?

self._balanceself.balance_

Matches

self.balance = 0 # meant to be protected

↓

The name has no leading underscore, so it is public. A protected attribute needs a single underscore: self._balance = 0.

Applying

Meanings

Term

_name

Meaning

A single underscore prefix marks a protected member: by convention it is for use inside the class and its subclasses only. Python does not enforce it, so outside code can still access it.

Practice

class Vehicle:

Testing

Gaps

= "Rex"

Check Answer

Spots

3 self._name = name

4class Dog:

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Quick Questions

1. Which attribute name marks a protected member in Python?

A. self._balance ✓

B. self.balance_

A protected member starts with a single underscore, like _balance. An underscore at the end has no special meaning.

2. Python stops outside code from reading a protected attribute such as obj._name.

A. True

B. False ✓

A single underscore is only a convention. Python does not enforce it, so outside code can still read obj._name.

3. A protected attribute _name is intended to be used...

A. Inside the class and its subclasses ✓

B. Anywhere in the program

The underscore tells other programmers that _name is for the class and its subclasses, not for general use.

4. Which subclass method uses a protected attribute from its parent class correctly?

A. def show(self): print(_name)

B. def show(self): print(self._name) ✓

A subclass accesses an inherited protected attribute through self, like self._name. Without self, Python looks for a variable called _name and fails.

5. Why do programmers use a single underscore on an attribute?

A. To signal it is not part of the public interface ✓

B. To make Python block access to it

The underscore is a signal to other programmers. It works by convention and does not enforce anything.

Match the Pairs

self._balance = 0 → Creates a protected attribute: the single underscore signals it is meant for use only inside the class and its subclasses

def _calculate_fee(self): → Defines a protected method that is intended to be called only by the class itself or its subclasses

A subclass can use a protected attribute inherited from its parent class → class Savings(Account): def show(self): print(self._balance)

Protected is a convention, not enforcement: code outside the class can still access the attribute → acc = Account() print(acc._balance)

self.balance = 0 # meant to be protected → The name has no leading underscore, so it is public. A protected attribute needs a single underscore: self._balance = 0.

print(acc._balance) # in main program → Code outside the class and its subclasses should not use a protected attribute directly. Use a public method or property such as acc.get_balance() instead.

Key Term

_name

A single underscore prefix marks a protected member: by convention it is for use inside the class and its subclasses only. Python does not enforce it, so outside code can still access it.

Fill in the Blanks

1. class Pet: _name = "Rex" age = 3

Hint: Type the attribute name that marks it as protected, using a single underscore prefix.

2. class Dog(Pet): def speak(self): return self._name + " barks"

Hint: Type the protected attribute inherited from the parent class that the subclass is allowed to use.

3. class Account: def _check(self): return self.balance > 0

Hint: Type the method name that makes this a protected method, using a single underscore prefix.

4. acc = Account(50) print(acc._balance) # Python allows this

Hint: Type the protected attribute being read from outside the class. Python only uses a convention, so it does not block this.

5. class Saver(Account): def show(self): print(self._check())

Hint: Type the protected method inherited from the parent class that the subclass calls.

Spot the Error

Question 1

A class Pet:

B def __init__(self, name):

C self._name = name

D class Dog:

E def show(self):

F print(self._name)

Line D — Dog must inherit from Pet to use the protected attribute _name. It should be class Dog(Pet): so subclass code can access self._name.

Question 2

A class Savings(Account)

B def add(self, amt):

C self._balance += amt

Line A — A class header must end with a colon. It should be class Savings(Account): so the subclass can use the protected _balance.

Question 3

A class Car:

B def __init__(self, make):

C self.make = make

D def info(self):

E return self._make

Line C — This creates a public attribute called make, not the protected _make. info() then fails because self._make does not exist. It should be self._make = make.

Question 4

A Class Staff:

B def __init__(self, pay):

C self._pay = pay

Line A — The class keyword must be lowercase in Python. It should be class Staff: before the protected attribute _pay is set.

Question 5

A class Student:

B def _init_(self, name):

C self._name = name

Line B — The constructor needs double underscores on each side: __init__. A single underscore only marks a protected member, so Python would not call this when an object is created.

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