Oppiskele How property Works Under the Hood

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property is one of the most commonly used Python built-ins. Most developers use it as a decorator without thinking about how it works. It is, in fact, a data descriptor implemented in C – and understanding it as a descriptor explains every behavior it has.

`property` as a Data Descriptor

property defines __get__, __set__, and __delete__, making it a data descriptor. When assigned as a class attribute, it intercepts all attribute access on instances:


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# property is a class – you can inspect it
print(type(property))           # <class 'type'>
print(hasattr(property, "__get__"))   # True
print(hasattr(property, "__set__"))   # True
print(hasattr(property, "__delete__")) # True

The Decorator Syntax Is Syntactic Sugar

The @property decorator is equivalent to calling property() directly with a getter function:


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# Using @property decorator
class Account:
    def __init__(self, balance):
        self._balance = balance

    @property
    def balance(self):
        return self._balance

# Equivalent without decorator syntax
class Account:
    def __init__(self, balance):
        self._balance = balance

    def _get_balance(self):
        return self._balance

    balance = property(_get_balance)  # Same result as @property

Adding a Setter and Deleter

property returns a new property object with each chained method – fget, fset, and fdel store the three functions:


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# Full property with getter, setter, and deleter
class BankAccount:
    def __init__(self, account_id, balance):
        self.account_id = account_id
        self._balance = balance

    @property
    def balance(self):
        return self._balance

    @balance.setter
    def balance(self, value):
        if not isinstance(value, (int, float)) or value < 0:
            raise ValueError(f"Balance must be non-negative, got {value!r}")
        self._balance = value

    @balance.deleter
    def balance(self):
        self._balance = 0.0
        print(f"Balance for account {self.account_id} reset to 0")

account = BankAccount("ACC-001", 5000.0)
print(account.balance)    # 5000.0

account.balance = 6500.0  # Calls setter
print(account.balance)    # 6500.0

del account.balance        # Calls deleter
print(account.balance)    # 0.0

What `@balance.setter` Actually Does

balance.setter(func) calls property.setter() on the existing property and returns a new property object with fset set. This is why each decorated method must use the same name:


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# Inspecting what property stores
class Invoice:
    def __init__(self, amount):
        self._amount = amount

    @property
    def amount(self):
        return self._amount

    @amount.setter
    def amount(self, value):
        self._amount = value

print(Invoice.amount)         # <property object at 0x...>
print(Invoice.amount.fget)    # <function Invoice.amount at 0x...>
print(Invoice.amount.fset)    # <function Invoice.amount at 0x...>
print(Invoice.amount.fdel)    # None – no deleter defined

`property` vs a Custom Descriptor

property is convenient for single-class use. A custom descriptor is better when the same validation logic applies to multiple attributes across multiple classes:

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How property Works Under the Hood

property as a Data Descriptor

The Decorator Syntax Is Syntactic Sugar

Adding a Setter and Deleter

What @balance.setter Actually Does

property vs a Custom Descriptor

`property` as a Data Descriptor

What `@balance.setter` Actually Does

`property` vs a Custom Descriptor