Demonstrate https://github.com/zopefoundation/persistent/issues/3. The first file shows that persistent.mapping.PersistentMapping does *not* call its base class '__init__()' via 'super()'; the second shows that persistent.Persistent does.

with_mapping.py

from persistent.mapping import PersistentMapping

class Base1(object):
    def __init__(self):
        super(Base1, self).__init__()
        print('Base1')

class Base2(object):
    def __init__(self):
        super(Base2, self).__init__()
        print('Base2')

class Base3(PersistentMapping):
    def __init__(self):
        super(Base3, self).__init__()
        print('Base3')

class Foo(PersistentMapping, Base1, Base2):
    def __init__(self):
        super(Foo, self).__init__()

class Bar(Base1, Base2, PersistentMapping):
    def __init__(self):
        super(Bar, self).__init__()

class Baz(PersistentMapping, Base1, Base2):
    def __init__(self):
        super(Baz, self).__init__()


print('Instantiating Foo')
print('-' * 50)
foo = Foo()
print('-' * 50)

print('Instantiating Bar')
print('-' * 50)
bar = Bar()
print('-' * 50)

print('Instantiating Baz')
print('-' * 50)
bar = Baz()
print('-' * 50)

with_persistent.py

from persistent import Persistent

class Base1(object):
    def __init__(self):
        super(Base1, self).__init__()
        print('Base1')

class Base2(object):
    def __init__(self):
        super(Base2, self).__init__()
        print('Base2')

class Base3(Persistent):
    def __init__(self):
        super(Base3, self).__init__()
        print('Base3')

class Foo(Persistent, Base1, Base2):
    def __init__(self):
        super(Foo, self).__init__()

class Bar(Base1, Base2, Persistent):
    def __init__(self):
        super(Bar, self).__init__()

class Baz(Persistent, Base1, Base2):
    def __init__(self):
        super(Baz, self).__init__()


print('Instantiating Foo')
print('-' * 50)
foo = Foo()
print('-' * 50)

print('Instantiating Bar')
print('-' * 50)
bar = Bar()
print('-' * 50)

print('Instantiating Baz')
print('-' * 50)
bar = Baz()
print('-' * 50)