106 lines
3.0 KiB
Python
106 lines
3.0 KiB
Python
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#!/usr/bin/python
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import weakref
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class Finalizable(object):
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"""
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Base class enabling the use a __finalize__ method without all the
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problems associated with __del__ and reference cycles.
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If you call enable_finalizer(), it will call __finalize__
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with a single "ghost instance" argument after the object has been
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deleted. Creation of this "ghost instance" does not involve calling
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the __init__ method, but merely copying the attributes whose names
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were given as arguments to enable_finalizer().
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A Finalizable can be part of any reference cycle, but you must be careful
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that the attributes given to enable_finalizer() don't reference back to
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self, otherwise self will be immortal.
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"""
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__wr_map = {}
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__wr_id = None
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def bind_finalizer(self, *attrs):
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"""
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Enable __finalize__ on the current instance.
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The __finalize__ method will be called with a "ghost instance" as
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single argument.
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This ghost instance is constructed from the attributes whose names
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are given to bind_finalizer(), *at the time bind_finalizer() is called*.
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"""
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cls = type(self)
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ghost = object.__new__(cls)
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ghost.__dict__.update((k, getattr(self, k)) for k in attrs)
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cls_wr_map = cls.__wr_map
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def _finalize(ref):
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try:
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ghost.__finalize__()
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finally:
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del cls_wr_map[id_ref]
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ref = weakref.ref(self, _finalize)
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id_ref = id(ref)
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cls_wr_map[id_ref] = ref
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self.__wr_id = id_ref
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def remove_finalizer(self):
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"""
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Disable __finalize__, provided it has been enabled.
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"""
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if self.__wr_id:
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cls = type(self)
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cls_wr_map = cls.__wr_map
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del cls_wr_map[self.__wr_id]
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del self.__wr_id
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class TransactionBase(Finalizable):
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"""
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A convenience base class to write transaction-like objects,
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with automatic rollback() on object destruction if required.
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"""
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finished = False
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def enable_auto_rollback(self):
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self.bind_finalizer(*self.ghost_attributes)
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def commit(self):
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assert not self.finished
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self.remove_finalizer()
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self.do_commit()
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self.finished = True
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def rollback(self):
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assert not self.finished
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self.remove_finalizer()
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self.do_rollback(auto=False)
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self.finished = True
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def __finalize__(ghost):
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ghost.do_rollback(auto=True)
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class TransactionExample(TransactionBase):
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"""
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A transaction example which close()s a resource on rollback
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"""
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ghost_attributes = ('resource', )
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def __init__(self, resource):
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self.resource = resource
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self.enable_auto_rollback()
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def __str__(self):
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return "ghost-or-object %s" % object.__str__(self)
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def do_commit(self):
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pass
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def do_rollback(self, auto):
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if auto:
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print "auto rollback", self
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else:
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print "manual rollback", self
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self.resource.close()
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