ex.py

class MyClass(object):
    _myvar = None
    
class MyOtherClass(object):
    pass

a = MyClass()
b = MyOtherClass()

a._myvar
-> None

b._myvar
-> Traceback (most recent call last):
     File "<input>", line 1, in <module>
   AttributeError: 'MyOtherClass' object has no attribute '_myvar'


dir(a)
-> ['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', '_myvar']
dir(b)
-> ['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__']

a._myvar = 1
a._myvar
-> 1
MyClass._myvar
-> None