Unpickling new-style with kwargs not possible?
During instantiation of my class, I initialize some fields that are not picklable. Thus, in order to be able to (un)pickle my classes correctly, I would like my init method to be called on unpickling. This is, it seems, the way it worked with old-style classes.
With new style classes, I need to use __new__
and __getnewargs__
. Here is what I do:
import cPickle
class Blubb(object):
def __init__(self, value):
self.value = value
class Bla(Blubb):
def __new__(cls, value):
instance = super(Bla, cls).__new__(cls)
instance.__开发者_如何学JAVAinit__(value)
return instance
def __getnewargs__(self):
return self.value,
def __getstate__(self):
return {}
def __setstate__(self, dct):
pass
x = Bla(2)
print x.value
pickled = cPickle.dumps(x, 2)
x_ = cPickle.loads(pickled)
assert x_.value == 2
This would be fine, if not for the fact that obj = C.__new__(C, *args)
. There is now **kwargs
. So I am restricted to non keyword arguments in my __new__
and __init__
methods.
Does anyone know of a way to solve this? This is really unconvenient.
The pickle protocol 2 wants to call cls.__new__(cls, *args)
by default, but there is a way around this. If you use __reduce__
you can return a function which will map your arguments to __new__
. I was able to modify your example to get **kwargs
to work:
import cPickle
class Blubb(object):
def __init__(self, value, foo=None, bar=None):
self.value = value
self.foo = foo
self.bar = bar
def _new_Bla(cls, value, kw):
"A function to map kwargs into cls.__new__"
return cls.__new__(cls, value, **kw)
class Bla(Blubb):
def __new__(cls, value, **kw):
instance = super(Bla, cls).__new__(cls)
instance.__init__(value, **kw)
return instance
def __reduce__(self):
kwargs = {'foo': self.foo, 'bar': self.bar}
return _new_Bla, (self.__class__, self.value, kwargs), None
x = Bla(2, bar=[1, 2, 3])
pickled = cPickle.dumps(x, 2)
y = cPickle.loads(pickled)
assert y.value == 2
assert y.bar == [1, 2, 3]
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