Problem with copy EmbeddedDocumentListField

I need copy EmbeddedDocumentListField and I have huge problem because when I use deepcopy I have got error AttributeError: _instance.

Simple example, of error, below:

 from copy import deepcopy
 
 from mongoengine import (
     Document,
     EmbeddedDocument,
     IntField,
     StringField,
     EmbeddedDocumentListField
 )
 
 
 class E(EmbeddedDocument):
     x = IntField(required=True)
     y = IntField(required=True)
 
     def __str__(self):
         return '{}:{}'.format(self.x, self.y)
 
 class A(Document):
     f = StringField(required=True, max_length=256)
     e = EmbeddedDocumentListField(E)
 
 
 a = A(f='a1')
 a.e.append(E(x=1, y=2))
 a.e.append(E(x=10, y=20))
 e_from_a = a.e

 e_copy = deepcopy(e_from_a)

print (type(e_copy) == type(e_from_a))

True

print (e_copy == e_from_a)

True

e_from_a[0]           

<E: 1:2>

But:

e_copy[0]

lib/python3.6/site-packages/mongoengine/base/datastructures.py in __getitem__(self, key, *args, **kwargs)
    116 
    117         EmbeddedDocument = _import_class('EmbeddedDocument')
--> 118         if isinstance(value, EmbeddedDocument) and value._instance is None:
    119             value._instance = self._instance
    120         elif not isinstance(value, BaseDict) and isinstance(value, dict):

AttributeError: _instance

I could copy element by element, but that's not what I mean.

Start by reproducing the deepcopy example and inspect mongoengine/base/datastructures.py at the getitem path shown in the traceback. Trace how EmbeddedDocumentListField and its embedded documents are copied; done means a deep-copied list preserves its type and equality and allows indexed access without AttributeError.