[Python-ideas] new pickle semantics/API (original) (raw)

tomer filiba tomerfiliba at gmail.com
Thu Jan 25 22:15:04 CET 2007

there's a bug in the copy function that i wanted to fix: def copy(obj): cls, state = obj.getstate() return cls.setstate(state)

also, but there's a reason why getstate returns "(cls, state)" rather than just "state", and that's to keep things agile. i don't want to be necessarily tightly-coupled to a certain type.

the cls will be used to reconstruct the object (cls.setstate), much like the function returned by reduce, so normally, getstate would just return self.class for cls.

but there are times, especially when object proxies are involved, that we may want to "lie" about the actual type, i.e., use a different type to reconstruct the object with.

here's an example that shows why: class ListProxy: ... def getstate(self): return list, self._value

instances of ListProxy, when stored in a file (i.e., shelf), want to be pickled by value. moreover, when they are loaded from a file, they want to loaded as actual lists, not proxies, as the proxied object is long lost.

so returning a tuple of (cls, state) gives more freedom to frameworks and other utilities. of course, again, most code would just return (self.class, state)

-tomer

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