>>> class A: ...
... 
>>> help(A)
Help on class A in module __main__:

class A(builtins.object)
 |  Data descriptors defined here:
 |
 |  __dict__
 |      dictionary for instance variables (if defined)
 |
 |  __weakref__
 |      list of weak references to the object (if defined)

>>> A.__dict__['__dict__'].__doc__
'dictionary for instance variables (if defined)'

>>> class B:
...    __slots__ = ()
... 
>>> help(B)
Help on class B in module __main__:

class B(builtins.object)

>>> B.__dict__['__dict__']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    B.__dict__['__dict__']
    ~~~~~~~~~~^^^^^^^^^^^^
KeyError: '__dict__'