Neal Becker wrote:
> I find pickle really handy for saving results from my (simulation)
> experiments. But recently I realized there is an issue. Reading the
> saved results requires loading the pickle, which in turn will load any
> referenced
> modules. Problem is, what if the modules have changed?
>
> For example, I just re-implemented a python module in C++, in a not quite
> compatible way. AFAIK, my only choice to not break my setup is to choose
> a different name for the new module.
>
> Has anyone else run into this issue and have any ideas? I can imagine
> perhaps some kind of module versioning could be used (although haven't
> really thought through the details).
I just tried the renaming part (replacing one.Foo with two.Bar):
$ cat upgrade.py
import pickle
import one
with open("data.dat", "wb") as f:
pickle.dump(one.Foo(1, 2), f)
with open("data.dat", "rb") as f:
print(pickle.load(f))
class MyUnpickler(pickle.Unpickler):
def find_class(self, module, name):
if (module, name) == ("one", "Foo"):
module, name = ("two", "Bar")
return super().find_class(module, name)
with open("data.dat", "rb") as f:
print(MyUnpickler(f).load())
$ python3.7 upgrade.py
Foo(alpha=1, beta=2)
Bar(alpha=1, beta=2, gamma=42)
In simple cases Bar.__setstate__() should be able to deal with changes in
object layout. I used
def __setstate__(self, namespace):
namespace.setdefault("gamma", 42)
vars(self).update(namespace)
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