Pickling set subclass raises unhashable type: 'list'

This code (using custom list subclass) works well for me:

import pickle

class Numbers(list):
    def __init__(self, *numbers: int) -> None:
        super().__init__()
        self.extend(numbers)

numbers = Numbers(12, 34, 56)
numbers.append(78)

numbers = pickle.loads(pickle.dumps(numbers, protocol=3))

but when I change the parent class to set:

import pickle

class Numbers(set):
    def __init__(self, *numbers: int) -> None:
        super().__init__()
        self.update(numbers)

numbers = Numbers(12, 34, 56)
numbers.add(78)

numbers = pickle.loads(pickle.dumps(numbers, protocol=3))

the code raises a TypeError with this traceback:

Traceback (most recent call last):
  File "test.py", line 11, in <module>
    numbers = pickle.loads(pickle.dumps(numbers, protocol=3))
  File "test.py", line 6, in __init__
    self.update(numbers)
TypeError: unhashable type: 'list'

The set subclass is successfully initialized and works well but trying to pickle it raises a very confusing exception since there is actually no list used in my code.

Solution

Summary

Modifying the constructor of a built-in type is hard and error-prone since other methods migth depend on it. Avoid whenever possible.

Error inspection

First, by forcing the Python implementation over the compiled C implementation of the pickle module we get more information in the traceback:

Traceback (most recent call last):
  File "test.py", line 14, in <module>
    numbers = pickle.loads(pickle.dumps(numbers, protocol=3))
  File "/usr/local/lib/python3.7/pickle.py", line 1604, in _loads
    encoding=encoding, errors=errors).load()
  File "/usr/local/lib/python3.7/pickle.py", line 1086, in load
    dispatch[key[0]](self)
  File "/usr/local/lib/python3.7/pickle.py", line 1437, in load_reduce
    stack[-1] = func(*args)
  File "test.py", line 6, in __init__
    self.update(numbers)
TypeError: unhashable type: 'list'

Although the documentation states:

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

we can see from the traceback above that the __init__ method is invoked if the pickled representation of the class contains the REDUCE opcode (generally when the class implements a custom __reduce__ method) and, as the inspection of the pickled representation shows, the REDUCE opcode is indeed present:

 0: \x80 PROTO      3
 2: c    GLOBAL     '__main__ Numbers'
20: q    BINPUT     0
22: ]    EMPTY_LIST
23: q    BINPUT     1
25: (    MARK
26: K        BININT1    56
28: K        BININT1    34
30: K        BININT1    12
32: K        BININT1    78
34: e        APPENDS    (MARK at 25)
35: \x85 TUPLE1
36: q    BINPUT     2
38: R    REDUCE
39: q    BINPUT     3
41: }    EMPTY_DICT
42: q    BINPUT     4
44: b    BUILD
45: .    STOP

Solution

Avoid modifying the constructor:

import pickle

class Numbers(set):
    pass

numbers = Numbers([12, 34, 56])
numbers.add(78)

numbers = pickle.loads(pickle.dumps(numbers, protocol=3))

or, if you really have to, at least make sure to pass all the arguments to the parent constructor:

import pickle

class Numbers(set):
    def __init__(self, *args, **kwargs) -> None:
        super().__init__(*args, **kwargs)

numbers = Numbers([12, 34, 56])
numbers.add(78)

numbers = pickle.loads(pickle.dumps(numbers, protocol=3))