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Migration instructions
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There are a number of backwards-incompatible changes. These points should help with migrating from an older release:
* The ``key_builder`` parameter for caches now expects a callback which accepts 2 strings and returns a string in all cache implementations, making the builders simpler and interchangeable.
* The ``key`` parameter has been removed from the ``cached`` decorator. The behaviour can be easily reimplemented with ``key_builder=lambda *a, **kw: "foo"``
* When using the ``key_builder`` parameter in ``multicached``, the function will now return the original, unmodified keys, only using the transformed keys in the cache (this has always been the documented behaviour, but not the implemented behaviour).
* ``BaseCache`` and ``BaseSerializer`` are now ``ABC``s, so cannot be instantiated directly.
* If subclassing ``BaseCache`` to implement a custom backend:
* The cache key type used by the backend must now be specified when inheriting (e.g. ``BaseCache[str]`` typically).
* The ``build_key()`` method must now be defined (this should generally involve calling ``self._str_build_key()`` as a helper).