[dataclasses] (correction) exo 2: use classvar instead of magic number

d_dataclasses/02_person.py

@@ -5,7 +5,8 @@ We want the Person dataclass to have 3 attribute: the name, last name, and age o
 But we want to be able to give the birthday (we limit ourselves to the year), and the class should determine the age on its own.
 """
 import doctest
-from dataclasses import dataclass, InitVar
+from dataclasses import InitVar, dataclass
+from typing import ClassVar
 
 
 @dataclass
@@ -26,10 +27,11 @@ class Person:
     age: int = None
 
     birthday: InitVar[int] = None
+    YEAR: ClassVar[int] = 2019
 
     def __post_init__(self, birthday: int):
         if self.age is None and birthday is not None:
-            self.age = 2019 - birthday
+            self.age = self.YEAR - birthday
 
 
 if __name__ == '__main__':