Setter methods that check new values

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. Which setter protects the private attribute _balance from a negative value?

A def set_balance(self, amount): self._balance = amount

B def set_balance(self, amount): if amount >= 0: self._balance = amount

2. What is the main job of a setter method that checks values?

A Change a private attribute only if the new value is valid

B Change a private attribute to whatever value it is given

3. Which line inside set_balance correctly rejects a bad value?

A if amount < 0: self._balance = amount

B if amount < 0: print("Invalid amount")

4. Account is an object with a private _balance. Which code uses the setter correctly from outside the class?

A acc.set_balance(150)

B acc.set_balance = 150

5. Which pair is a getter and setter for _balance?

A get_balance(self) returns self._balance; set_balance(self, amount) changes it

B get_balance(self) changes self._balance; set_balance(self, amount) returns it

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1def set_balance(self, amount):		ADefines a setter method that receives a new value for the private balance attribute so it can be checked before it is stored
2if amount >= 0: self._balance = amount		Bdef get_balance(self): return self._balance
3A setter can reject a bad value by refusing to change the attribute and giving feedback instead		CChecks the new value is valid and only then updates the private attribute
4A getter and setter pair lets code read a private attribute and change it safely		Delse: print("Invalid amount")
5def set_balance(self, amount): self._balance = amount		EThe value is saved to a local variable called balance, not to self._balance, so the object's attribute never changes
6def set_balance(self, amount): if amount >= 0: balance = amount		FThe setter stores the value without checking it, so a negative amount could be saved and the object would hold bad data

Definitions

def set_balance(self, amount):

Fill in the Blanks

Complete each line of code by filling in the missing parts.

def ___________(self, amount): if amount >= 0: self._balance = amount

__ amount >= 0: self._balance = amount

if amount _ 0: return

def ___________(self): return self._balance

self.________ = amount

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A def set_balance(self, amount):

B if amount >= 0:

C self.balance = amount

D else:

E print('Invalid')

Question 2

A def get_balance(self):

B return _balance

C def set_balance(self, amount):

D if amount > 0:

E self._balance = amount

Question 3

A def set_balance(self, amount):

B if amount >= 0:

C amount = self._balance

Question 4

A def set_balance(amount):

B if amount < 0:

C print('Bad amount')

D else:

E self._balance = amount

Question 5

A def set_balance(self amount):

B if amount >= 0:

C self._balance = amount

D return True

E return False

Answer Key

Setter methods that check new values

Section A: Quick Questions

B — def set_balance(self, amount): if amount >= 0: self._balance = amount

A — Change a private attribute only if the new value is valid

B — if amount < 0: print("Invalid amount")

A — acc.set_balance(150)

A — get_balance(self) returns self._balance; set_balance(self, amount) changes it

Section B: Match the Pairs

def set_balance(self, amount): → Defines a setter method that receives a new value for the private balance attribute so it can be checked before it is stored

if amount >= 0: self._balance = amount → Checks the new value is valid and only then updates the private attribute

A setter can reject a bad value by refusing to change the attribute and giving feedback instead → else: print("Invalid amount")

A getter and setter pair lets code read a private attribute and change it safely → def get_balance(self): return self._balance

def set_balance(self, amount): self._balance = amount → The setter stores the value without checking it, so a negative amount could be saved and the object would hold bad data

def set_balance(self, amount): if amount >= 0: balance = amount → The value is saved to a local variable called balance, not to self._balance, so the object's attribute never changes

Section C: Definitions

def set_balance(self, amount): — A setter method that changes a private attribute only after checking the new value is valid, such as rejecting a negative amount, so the object never holds bad data.

Section D: Fill in the Blanks

def set_balance(self, amount):
    if amount >= 0:
        self._balance = amount

if amount >= 0:
    self._balance = amount

if amount < 0:
    return

def get_balance(self):
    return self._balance

self._balance = amount

Section E: Spot the Error

C — The setter must update the private attribute self._balance. Writing to self.balance creates a different public attribute, so the checked value is never stored in the right place.

B — The getter must use self to reach the attribute. It should be return self._balance, because _balance on its own is not a defined variable.

C — The assignment is the wrong way round. The valid new value must be stored in the attribute: self._balance = amount.

A — Every method in a class needs self as its first parameter. Without it the setter cannot reach self._balance. It should be def set_balance(self, amount):.

A — Parameters must be separated by a comma. It should be def set_balance(self, amount):.