Question

I just wonder, why variable defined by __init__ is not accessible from class? Should its executed during instantiation, so that its accessible from outside?

>>> class a:
...     data = {}
... 
>>> a.data
{}
>>> class a:
...     def __init__(self):
...             self.data = {}
... 
>>> a.data
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: class a has no attribute 'data'

Answer 1

Variables defined in __init__ are instance variables, by definition they can’t be accessed from a class scope. That’s why this works:

a().data # data is a member of an instance of class `a`

… Whereas this won’t work:

a.data   # data is not a member of the `a` class

Notice that __init__ (the initializer) is run when you create a new instance of a by calling a(), from that point on all the variables defined in __init__ are bound to that particular instance of a.