Demonstrate how __getattr__ and __setattr__ interact with slots and normal properties. The interesting part is that if a slot is not assigned a value, __getattr__ will be called for it as for normal attributes.

slot-test.py

# Demonstrate when __getattr__ and __setattr__ get called for slots
# and normal attributes.
#
# The output is:
#     a = Main(42)
#        __setattr__(abc, 42) called
#     a.abc: 42
#        __getattr__(cba) called
#     a.cba: None
#        __getattr__(slot) called
#     a.slot: None
#     a.slot = 69
#        __setattr__(slot, 69) called
#     a.slot: 69
#     a.__dict__: {'abc': 42}

from __future__ import print_function


class Base(object):
    __slots__ = ('slot',)


class Main(Base):
    def __init__(self, abc):
        self.abc = abc

    def __getattr__(self, name):
        print('   __getattr__({}) called'.format(name))
        return None

    def __setattr__(self, name, value):
        print('   __setattr__({}, {}) called'.format(name, value))
        Base.__setattr__(self, name, value)


if __name__ == '__main__':
    print('a = Main(42)')
    a = Main(42)
    print('a.abc:', a.abc)
    print('a.cba:', a.cba)
    print('a.slot:', a.slot)
    print('a.slot = 69')
    a.slot = 69
    print('a.slot:', a.slot)
    print('a.__dict__:', a.__dict__)