Protect book data.
Keep valid state.

Mini Library Manager now validates a book’s publication year through a clear property interface.

Learning goalUse _year, @property, and a setter to validate a positive integer year without leaving a Book in an invalid state.

01 Learn02 Try03 Predict

01 Learn

Control how a book changes.

Encapsulation groups data with the rules that protect it. In Python, a leading underscore such as _year is a convention for internal use, not a strict access ban.

class Book:
    def __init__(self, title, author, year):
        self.title = title
        self.author = author
        self.is_available = True
        self.year = year

    @property
    def year(self):
        return self._year

    @year.setter
    def year(self, value):
        if not isinstance(value, int) or value <= 0:
            raise ValueError("year must be a positive integer")
        self._year = value

    def borrow(self):
        self.is_available = False

    def return_book(self):
        self.is_available = True

book = Book("Dune", "Frank Herbert", 1965)
try:
    book.year = 0
except ValueError:
    pass
print(book.year)

self.year = year calls the setter during creation, so validation also protects the initial value. The setter checks before assigning self._year.

1965

The failed assignment raises ValueError before _year changes. After catching the error, the earlier valid year remains 1965.

↳

Property pattern: Read book.year through the getter. Assign book.year = value through the setter. Keep _year as the internal stored value.

Validate before changing state

class Book:
    def __init__(self, title, author, year):
        self.title = title
        self.author = author
        self.is_available = True
        self.year = year

    @property
    def year(self):
        return self._year

    @year.setter
    def year(self, value):
        if not isinstance(value, int) or value <= 0:
            raise ValueError("year must be a positive integer")
        self._year = value

    def borrow(self):
        self.is_available = False

    def return_book(self):
        self.is_available = True

book = Book("Dune", "Frank Herbert", 1965)
try:
    book.year = 0
except ValueError:
    pass
print(book.year)

The public year property is easy to use, while its setter rejects zero, negative, or non-integer years.

A careful setter

@year.setter
def year(self, value):
    if not isinstance(value, int) or value <= 0:
        raise ValueError("year must be a positive integer")
    self._year = value

Validate first and assign second. That order means invalid input cannot overwrite a valid stored value.

Follow a protected update

1. Use the public propertybook.year = 1965 calls the setter instead of writing directly to _year.
2. Validate the new valueThe setter accepts only positive integers and raises ValueError otherwise.
3. Store after validationOnly a valid value reaches self._year.
4. Keep the earlier stateA failed assignment leaves the book’s previously valid year unchanged.

Common mistakes

Writing self.year inside the setterUse self._year in the setter; assigning self.year there calls the setter again.
Assigning before validationCheck the value before changing _year so invalid state is never stored.
Treating _year as private enforcementThe underscore is a convention that communicates internal use; Python does not block direct access.
Forgetting validation in __init__Assign self.year = year in __init__ so creation uses the same setter rule.

02 Try

Python Playground

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Try a valid year update, then try an invalid value inside try/except and inspect the saved year.

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03 Predict

What will this program print?

The setter rejects zero before it can replace the existing valid year.

class Book:
    def __init__(self, title, author, year):
        self.title = title
        self.author = author
        self.is_available = True
        self.year = year

    @property
    def year(self):
        return self._year

    @year.setter
    def year(self, value):
        if not isinstance(value, int) or value <= 0:
            raise ValueError("year must be a positive integer")
        self._year = value

    def borrow(self):
        self.is_available = False

    def return_book(self):
        self.is_available = True

book = Book("Dune", "Frank Herbert", 1965)
try:
    book.year = 0
except ValueError:
    pass
print(book.year)

Practice independently

Mini Challenge — Validate a rating

Add one simple validated property to a Book without bypassing its internal state.

Your goalCreate a rating property that accepts only integers from 1 to 5 and raises ValueError for other values.

rating_property_challenge.py

Optional hints

Use @property above def rating(self), then @rating.setter above def rating(self, value).
Check isinstance(value, int) and 1 <= value <= 5 before assigning self._rating.

Ask Sloby for a hint