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rF t|�r4t�| |� Y S t�| � Y S Y n:X |dk r�t| d�spt| d�sptd| ��t| |d�}|S dS )a� Return a *safe* weak reference to a callable target.
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method reference, will create a BoundMethodWeakref, otherwise
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object and the function which together define the instance method.
Attributes:
- ``key``: The identity key for the reference, calculated by the
class's calculate_key method applied to the target instance method.
- ``deletion_methods``: Sequence of callable objects taking single
argument, a reference to this object which will be called when
*either* the target object or target function is garbage
collected (i.e. when this object becomes invalid). These are
specified as the on_delete parameters of safe_ref calls.
- ``weak_self``: Weak reference to the target object.
- ``weak_func``: Weak reference to the target function.
Class Attributes:
- ``_all_instances``: Class attribute pointing to all live
BoundMethodWeakref objects indexed by the class's
calculate_key(target) method applied to the target objects.
This weak value dictionary is used to short-circuit creation so
that multiple references to the same (object, function) pair
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