subclass of `Box` does not unpickle correctly

[reidpr]

Steps to reproduce

Execute this program:

import box
import pickle

class MyBox(box.Box):
   ...

b_in = MyBox()
b_in.a = dict()
print("in root:  %s" % type(b_in))
print("in .a:    %s" % type(b_in.a))

p = pickle.dumps(b_in, protocol=pickle.HIGHEST_PROTOCOL)

b_out = pickle.loads(p)
print("out root: %s" % type(b_out))
print("out .a:   %s" % type(b_out.a))

Expected behavior

Output:

in root:  <class '__main__.MyBox'>
in .a:    <class '__main__.MyBox'>
out root: <class '__main__.MyBox'>
out .a:   <class '__main__.MyBox'>

Actual behavior

Output:

in root:  <class '__main__.MyBox'>
in .a:    <class '__main__.MyBox'>
out root: <class '__main__.MyBox'>
out .a:   <class 'box.box.Box'>

i.e., the type of key a is box.box.Box when it should be __main__.MyBox.

Comments

1. I believe the problem lies in this line:

   Box/box/box.py

   Line 236 in a4c10e9

   "box_class": box_class if box_class is not None else Box,

   where Box should insted be cls.

2. Instantiating b_in with box_class=MyBox does not help, I assume because there is something about class instantiation I don’t understand. However it did seem like it should work.

3. This does, however, work around the bug:

   class MyBox(box.Box):
      def __new__(cls, *args, **kwargs):
         kwargs["box_class"] = MyBox
         return super().__new__(cls, *args, **kwargs)