Python Name Mangling

class Person(object):
     def __init__(self, name, age):
         self.name = name
         self._age = age if age >= 0 else 0
         self.________a_ = 10
     
     @property
     def age(self):
         return self._age
     
    #  @age.setter
    #  def age(self, age):
    #      if age >= 0:
    #          self._age = age
    #      else:
    #          self._age  = 0

dir(a)

['_Person________a_',
 '__class__',
 '__delattr__',
 '__dict__',
 '__dir__',
 '__doc__',
 '__eq__',
 '__format__',
 '__ge__',
 '__getattribute__',
 '__gt__',
 '__hash__',
 '__init__',
 '__init_subclass__',
 '__le__',
 '__lt__',
 '__module__',
 '__ne__',
 '__new__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__subclasshook__',
 '__weakref__',
 '_age',
 'age',
 'name']

a = Person('Bob', 20)

a.age

20

a.age = 21

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/tmp/ipykernel_789/926215696.py in <module>
----> 1 a.age = 21

AttributeError: can't set attribute