Python: Why public and private instance attribute behave differently with property methods

Question

1. Using self.cost = cost in __init__ ,we are getting following output

   inside __init__

   inside setter

   inside property

   100

2. Using self._cost = cost in __init__ ,we are getting following output

   inside __init__

   inside property

   100

   In the first point,inside setter is being called but not in point 2.

    class Book(object): def __init__(self,cost): print('inside __init__') self.cost = cost #self._cost = cost @property def cost(self): print('inside property') return self._cost @cost.setter def cost(self,value): print('inside setter') self._cost = value book = Book(100) print(book.cost) 

Answer 1

It's not private vs. public, but your property name is cost so self.cost = cost triggers the property setter, but self._cost does not, because there is no property _cost . It will just assign new attribute _cost .

Answer 2

Hope this code makes it clear for you. There are few things to be considered, the decorator name should match exactly with the member variable cost or _cost . Also, the return should be _variablename . So if your variable name is _cost , you must return __cost .

Here is the small code example.

class Book_(object):
def __init__(self,cost):
    print('inside __init__')
    self._cost = cost

    @property
    def _cost(self):
        print('inside property')
        return self.__cost

    @_cost.setter
    def _cost(self,value):
        print('inside setter')
        self.__cost = value

class Book(object):
    def __init__(self,cost):
        print('inside __init__')
        self.cost = cost

    @property
    def cost(self):
        print('inside property')
        return self._cost   ## see the difference with above

    @cost.setter ## see the difference with above
    def cost(self,value):
        print('inside setter')
        self._cost = value    ## see the difference with above
book = Book(10)
print(book.cost)
print('---')
book2 = Book_(100)
print(book2._cost)

Output:

inside __init__
inside setter
inside property
10
---
inside __init__
inside setter
inside property
100

---