Unpickling an object of a user class that derives from list seems to miss calling the user class's __init__() method:

Consider this script, which defines a derived class of the built-in list for the purpose of creating a case insensitive list. The real example has all methods of list implemented, but this subset example also shows the issue:

-----------------
import pickle

def _lc_list(lst):
    result = list()
    for value in lst:
        result.append(value.lower())
    return result

class NocaseList(list):

    #def __new__(cls, iterable=()):
    #    self = super(NocaseList, cls).__new__(cls, iterable)
    #    print("Debug: __new__()  for self={}".format(id(self)))
    #    return self

    def __init__(self, iterable=()):
        print("Debug: __init__() for self={}".format(id(self)))
        super(NocaseList, self).__init__(iterable)
        self.lc_list = _lc_list(self)

    def append(self, value):
        super(NocaseList, self).append(value)
        self.lc_list.append(value.lower())

    def extend(self, iterable):
        super(NocaseList, self).extend(iterable)
        for value in iterable:
            self.lc_list.append(value.lower())

ncl = NocaseList(['A', 'b'])
pkl = pickle.dumps(ncl)
nclx = pickle.loads(pkl)
-----------------

$ python ./pickle_extend.py 
Debug: __init__() for self=4498704880
Traceback (most recent call last):
  File "./pickle_extend.py", line 32, in <module>
    nclx = pickle.loads(pkl)
  File "./pickle_extend.py", line 28, in extend
    self.lc_list.append(value.lower())
AttributeError: 'NocaseList' object has no attribute 'lc_list'

Uncommenting the __new__() method in the script shows that __new__() is called but not __init__():

$ python ./pickle_extend.py 
Debug: __new__()  for self=4359683024
Debug: __init__() for self=4359683024
Debug: __new__()  for self=4360134304
Traceback (most recent call last):
  File "./pickle_extend.py", line 32, in <module>
    nclx = pickle.loads(pkl)
  File "./pickle_extend.py", line 28, in extend
    self.lc_list.append(value.lower())
AttributeError: 'NocaseList' object has no attribute 'lc_list'

From https://docs.python.org/3/library/pickle.html#pickling-class-instances:

    When a class instance is unpickled, its __init__() method is usually not invoked.

If you want to change this behavior you have to implement the __reduce__ or __reduce_ex__ methods or register the object type in the global or per-pickler dispatch table. For example:

class NocaseList(list):

    def __reduce__(self):
        return self.__class__, (), None, iter(self)

Thanks for the clarification.

Just for the record:

I have implemented __setstate__() such that it completely restores the state from just the inherited list state. That makes it independent of whether __init__() or __new__() is called:

    def __getstate__(self):
        state = self.__dict__.copy()
        del state['_lc_list']
        return state

    def __setstate__(self, state):
        self.__dict__.update(state)
        self._lc_list = _lc_list(self)

This issue can be closed.

And just to be complete: That did not require implementing __reduce__().

this looks resolved?