Serhiy Storchaka <storchaka+cpyt...@gmail.com> added the comment:
>From PEP 307:
listitems Optional, and new in this PEP.
If this is not None, it should be an iterator (not a
sequence!) yielding successive list items. These list
items will be pickled, and appended to the object using
either obj.append(item) or obj.extend(list_of_items).
This is primarily used for list subclasses, but may
be used by other classes as long as they have append()
and extend() methods with the appropriate signature.
(Whether append() or extend() is used depends on which
pickle protocol version is used as well as the number
of items to append, so both must be supported.)
It says that obj.extend(list_of_items) should be called, not list.extend(obj,
list_of_items). Changing it now can break classes which rely on overridden
extend(). The special check in the C code is only for optimization, we can
inline extend() if it was not overridden.
Unfortunately __setstate__() is called after extend(), so there is no way to
set the validator in FieldList before it will be used in extend().
As a clever hack (if you do not want to do runtime check in every call of
extend()) you can set
self.extend = lambda items: list.extend(self, items)
in the __new__ method and add
del self.extend
in __init__ and __setstate__.
----------
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue43946>
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