property decorator

Topics

• decorators
• oops

The concept of setters and getters can be applied in Python with the help of @property:

class Resistor:
    def __init__(self, ohms):
        self.ohms = ohms
 
 
class BoundedResistance(Resistor):
    def __init__(self, ohms):
        super().__init__(ohms)
 
    @property
    def ohms(self):
        return self._ohms
 
    @ohms.setter
    def ohms(self, ohms):
        if ohms <= 0:
            raise ValueError(f"ohms must be > 0; found: {ohms}")
        self._ohms = ohms
 
r = BoundedResistance(1e3)
r.ohms = 0 # raises ValueError ...

Note

In the above example, doing r2 = BoundedResistance(-5) also raises ValueError, because BoundedResistance.__init__ calls Resistor.__init__, which assigns self.ohms = -5. That assignment causes the @ohms.setter method from BoundedResistance to be called, and it immediately runs the validation code before object construction has completed.

Few guidelines to keep in mind when use @property setters and getters:

• Do not set stuff in getter property methods
• Do not create unexpected side-effects such as importing modules dynamically, running slow helper functions, doing I/O, or making expensive database queries, as the users of a class will expect its attributes to be like any other Python object: quick and easy

Related

• descriptors in python