subclass of `Box` does not unpickle correctly

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
70/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Quiet
Tech stack
python
Domain
tooling

Research direction

Run the reproduction program from the issue, then inspect box/box.py around line 236 where nested boxes are constructed during unpickling. Confirm that a Box subclass remains the type of nested values after pickle.dumps and pickle.loads; done means the reported output shows MyBox for both the root and key a.

Written by the indexing model from the issue text.

Description

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:

    https://github.com/cdgriffith/Box/blob/a4c10e977b574114613431394b30412b50aaacce/box/box.py#L236

    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)
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