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# Copyright (C) 2020 Canonical Ltd.
#
# Author: Daniel Watkins <oddbloke@ubuntu.com>
#
# This file is part of cloud-init. See LICENSE file for license information.
class CloudInitPickleMixin:
"""Scaffolding for versioning of pickles.
This class implements ``__getstate__`` and ``__setstate__`` to provide
lightweight versioning of the pickles that are generated for classes which
use it. Versioning is done at the class level.
The current version of a class's pickle should be set in the class variable
``_ci_pkl_version``, as an int. If not overriden, it will default to 0.
On unpickle, the object's state will be restored and then
``self._unpickle`` is called with the version of the stored pickle as the
only argument: this is where classes should implement any deserialization
fixes they require. (If the stored pickle has no version, 0 is passed.)
"""
_ci_pkl_version = 0
def __getstate__(self):
"""Persist instance state, adding a pickle version attribute.
This adds a ``_ci_pkl_version`` attribute to ``self.__dict__`` and
returns that for serialisation. The attribute is stripped out in
``__setstate__`` on unpickle.
The value of ``_ci_pkl_version`` is ``type(self)._ci_pkl_version``.
"""
state = self.__dict__.copy()
state["_ci_pkl_version"] = type(self)._ci_pkl_version
return state
def __setstate__(self, state: dict) -> None:
"""Restore instance state and handle missing attributes on upgrade.
This will be called when an instance of this class is unpickled; the
previous instance's ``__dict__`` is passed as ``state``. This method
removes the pickle version from the stored state, restores the
remaining state into the current instance, and then calls
``self._unpickle`` with the version (or 0, if no version is found in
the stored state).
See https://docs.python.org/3/library/pickle.html#object.__setstate__
for further background.
"""
version = state.pop("_ci_pkl_version", 0)
self.__dict__.update(state)
self._unpickle(version)
def _unpickle(self, ci_pkl_version: int) -> None:
"""Perform any deserialization fixes required.
By default, this does nothing. Classes using this mixin should
override this method if they have fixes they need to apply.
``ci_pkl_version`` will be the version stored in the pickle for this
object, or 0 if no version is present.
"""
# vi: ts=4 expandtab
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