The @property Decorator and Setters

15 min · Intermediate

Most Python classes start with plain attributes such as item.price. When a rule appears later, for example "price must be positive", @property lets you add that rule while callers keep writing the same item.price. No get_price() or set_price() is needed.

Defining a Property Getter

Property is a method marked with @property that callers use like an attribute, without parentheses. Python runs the method each time the name is read.

Use @property above a method to allow it to be accessed like an attribute without parentheses.

Python

1class Circle:2    def __init__(self, radius):3        self.radius = radius4
5    @property6    def area(self):7        return 3.14159 * (self.radius ** 2)8
9c = Circle(5)10print(c.area)  # Accessed like an attribute, not c.area()

Example 1 · Computed order total

Python

1class OrderLine:2    def __init__(self, unit_price, qty):3        self.unit_price = unit_price4        self.qty = qty5
6    @property7    def total(self):8        return self.unit_price * self.qty9
10line = OrderLine(250, 3)11print(line.total)12line.qty = 513print(line.total)14try:15    line.total = 116except AttributeError:17    print("total is read-only")

Output

750
1250
total is read-only

What this code does

total is not stored, so it is always in step with qty.
With no setter, assigning to it raises AttributeError.

area is a read-only computed property. If a caller tries c.area = 50, Python raises an AttributeError.

Adding a Setter with Validation

Setter is a method marked @name.setter that runs when code assigns to the property, such as item.price = 10. It is where the rules for new values are checked.

To make a property writable, define a second method with the exact same name decorated with @<property_name>.setter.

Python

1class Temperature:2    def __init__(self, celsius=0.0):3        self.celsius = celsius  # Calls setter!4
5    @property6    def celsius(self):7        return self._celsius8
9    @celsius.setter10    def celsius(self, value):11        if value < -273.15:12            raise ValueError("Temperature below absolute zero is impossible!")13        self._celsius = float(value)

Example 2 · Setter that protects a price

Python

1class Product:2    def __init__(self, name, price):3        self.name = name4        self.price = price5
6    @property7    def price(self):8        return self._price9
10    @price.setter11    def price(self, value):12        if value <= 0:13            raise ValueError("price must be positive")14        self._price = value15
16p = Product("Monitor", 9000)17p.price = 850018print(p.price)19try:20    p.price = 021except ValueError as e:22    print(e)23print(p.price)

Output

8500
price must be positive
8500

What this code does

__init__ assigns self.price, so even the first value passes through the setter.
A rejected value leaves the old price in place.

The setter guards the internal _celsius storage. Invalid assignments are caught immediately.

Backward Compatibility Superpower

If your class started with simple self.celsius and other code accessed it directly, converting to @property requires zero changes in client code!

Callers continue writing t.celsius = 25 and reading t.celsius.
Validation occurs transparently behind the scenes.
No need for unsightly get_celsius() and set_celsius() methods.

Deleters with @<property>.deleter

You can also intercept attribute deletion using @<property_name>.deleter.

Python

1class FileRecord:2    @property3    def handle(self):4        return self._handle5    @handle.deleter6    def handle(self):7        print("Closing file handle before deletion")8        self._handle.close()

Example 3 · Deleter runs on del

Python

1class Connection:2    def __init__(self, host):3        self._host = host4
5    @property6    def host(self):7        return self._host8
9    @host.deleter10    def host(self):11        print("closing", self._host)12        self._host = None13
14c = Connection("db.internal")15del c.host16print(c.host)

Output

closing db.internal
None

What this code does

del c.host calls the deleter, which closes and clears the value.
Deleters are rare. Use them only when removing a value needs real clean-up.

Running del record.handle triggers the deleter method.

Next step

Check your understanding

12 quick questions

1class Circle:2 def __init__(self, radius):3 self.radius = radius4 5 @property6 def area(self):7 return 3.14159 * (self.radius ** 2)8 9c = Circle(5)10print(c.area) # Accessed like an attribute, not c.area()

1class OrderLine:2 def __init__(self, unit_price, qty):3 self.unit_price = unit_price4 self.qty = qty5 6 @property7 def total(self):8 return self.unit_price * self.qty9 10line = OrderLine(250, 3)11print(line.total)12line.qty = 513print(line.total)14try:15 line.total = 116except AttributeError:17 print("total is read-only")

1class Temperature:2 def __init__(self, celsius=0.0):3 self.celsius = celsius # Calls setter!4 5 @property6 def celsius(self):7 return self._celsius8 9 @celsius.setter10 def celsius(self, value):11 if value < -273.15:12 raise ValueError("Temperature below absolute zero is impossible!")13 self._celsius = float(value)

1class Product:2 def __init__(self, name, price):3 self.name = name4 self.price = price5 6 @property7 def price(self):8 return self._price9 10 @price.setter11 def price(self, value):12 if value <= 0:13 raise ValueError("price must be positive")14 self._price = value15 16p = Product("Monitor", 9000)17p.price = 850018print(p.price)19try:20 p.price = 021except ValueError as e:22 print(e)23print(p.price)

1class FileRecord:2 @property3 def handle(self):4 return self._handle5 @handle.deleter6 def handle(self):7 print("Closing file handle before deletion")8 self._handle.close()

1class Connection:2 def __init__(self, host):3 self._host = host4 5 @property6 def host(self):7 return self._host8 9 @host.deleter10 def host(self):11 print("closing", self._host)12 self._host = None13 14c = Connection("db.internal")15del c.host16print(c.host)