Alexandre Vassalotti added the comment:
I am currently fleshing out an improved implementation for the reduce protocol
version 4. One thing I am curious about is whether we should keep the special
cases we currently have there for dict and list subclasses.
I recall Raymond expressed disagreement in #msg83098 about this behavior. I
agree that having __setitem__ called before __init__ make it harder for dict
and list subclasses to support pickling. To take advantage of the special case,
subclasses need to do their required initialization in the __new__ method.
On the other hand, it does decrease the memory requirements for unpickling such
subclasses---i.e., we can build the object in-place instead of building an
intermediary list or dict. Reading PEP 307 confirms indeed that was the
original intention.
One possible solution, other than removing the special case completely, is to
make sure we initialize the object (using the BUILD opcode) before we call
__setitem__ or append on it. This would be a simple change that would solve the
initialization issue. However, I would still feel uneasy about the default
object.__reduce__ behavior depending on the object's subtype.
I think it could be worthwhile to investigate a generic API for pickling
collections in-place. For example, a such API would helpful for pickling set
subclasses in-place.
__items__() or Return an iterator of the items in the collection. Would be
__getitems__() equivalent to iter(dict.items()) on dicts and iter(list) on
lists.
__additems__(items) Add a batch of items to the collection. By default, it
would
be defined as:
for item in items:
self.__additem__(item)
However, subclasses would be free to provide a more
efficient
implementation of the method. Would be equivalent to
dict.update on dicts and list.extend on lists.
__additem__(item) Add a single item to the collection. Would be equivalent to
dict[item[0]] = item[1] on dicts and list.append on lists.
The collections module's ABCs could then provide default implementations of
this API, which would give its users efficient in-place pickling automatically.
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