Computed attributes
Expose a computed value without changing the public access syntax.
Python · Decorator
property
The property decorator exposes method-based behavior through normal attribute syntax. It can provide controlled reading, writing, deletion, and documentation for an attribute.
How it works
property creates a managed attribute from optional getter, setter, deleter, and documentation functions. Decorating a method with @property supplies read access; its .setter and .deleter helpers add assignment and deletion behavior while callers use ordinary attribute syntax.
Syntax
property(fget=None, fset=None, fdel=None, doc=None)Parameters
| Name | Type | Description |
|---|---|---|
fget | callable | None | Function called to retrieve the attribute value; defaults to None. |
fset | callable | None | Function called to assign an attribute value; defaults to None. |
fdel | callable | None | Function called to delete the attribute value; defaults to None. |
doc | str | None | Documentation string for the attribute; defaults to None. |
Returns
A property attribute.
Key points
- Calling property produces a descriptor that mediates access to an attribute.
- The fget callable supplies the behavior for reading the managed attribute.
- The fset callable receives assignments and can validate or transform incoming values.
- The fdel callable handles deletion of the managed attribute.
- The doc argument can define documentation for the property.
Example: Validated attribute access
class User:
def __init__(self, name):
self._name = name
@property
def name(self):
return self._name
@name.setter
def name(self, value):
if not value:
raise ValueError("name cannot be empty")
self._name = value
user = User("Ada")
print(user.name)
user.name = "Grace"
print(user.name)The getter exposes name as an attribute, while the setter validates assignments.
When to use it
Validation
Reject invalid assignments before storing state.
Encapsulation
Preserve an attribute-like API while changing internal representation.
Interview questions
QWhat does property do?
property is a built-in class that returns a property attribute. Its getter, setter, and deleter functions control reads, writes, and deletes.
Follow-up: How do you add write behavior after using @property? Define a method with the same property name and decorate it with @name.setter.
QWhat are property’s arguments?
fget is called when the attribute is read, fset when it is assigned, and fdel when it is deleted. The doc argument supplies the attribute documentation.
Follow-up: What happens if fset is omitted? Assignment through the property raises AttributeError.
QHow is property commonly used as a decorator?
The decorator form uses @property on a method to create the getter, then uses the returned property's setter or deleter decorators for optional mutation behavior. The underlying storage is commonly kept under a different instance attribute name.
Follow-up: Why use a separate backing attribute? It prevents the getter or setter from recursively referring to the property itself.
Related
property.setterproperty.deleterproperty.getterAttributeErrordescriptor