Python : Behavior of class and instance variables

Python : Behavior of class and instance variables

I have following two code samples
Example 1:
class MyClass(object):
def __init__(self, key, value):
self._dict = self._dict.update({key:value})
m = MyClass('ten',10)
print m._dict
Output:
AttributeError: 'MyClass' object has no attribute '_dict'
Example2:
class MyClass(object):
_dict = {}
def __init__(self, key, value):
self._dict = self._dict.update({key:value})
m = MyClass('ten',10)
print m._dict
Output: None
I am quite surprised with above behavior
Why the example2 compiled successfully by just addition of _dict = {}
line, and line present at class scope. also why None output? I believed
class scope variables has no relation with instance variable (special with
self)
Any Explaination?