Python __slot__ Attribute

Cited from the book "Python How to Program"

"""

When a new class defines the __slots__ attribute, objects of the class can assign values only to attributes whose names appear in the __slots__ list. If a client attempts to assign a value to an attribute whose name does not appear in __slots__, Python raises an exception.

If a new class defines attribute __slots__, but the class’s constructor does not initialize
the attributes’ values, Python assigns None to each attribute in __slots__ when an object
of the class is created.

A derived class inherits its base-class __slots__ attribute. However, if programs should
not be allowed to add attributes to objects of the derived class, the derived class must define
its own __slots__ attribute. The derived-class __slots__ contains only the allowed
derived-class attribute names, but clients still can set values for attributes specified by the
derived class’s direct and indirect bases classes.
"""

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Cited from Usage of __slot__

class Base:
    __slots__ = 'foo', 'bar'

class Right(Base):
    __slots__ = 'baz', 

class Wrong(Base):
    __slots__ = 'foo', 'bar', 'baz'        # redundant foo and bar