pickle is a standard library for serializing Python objects into bytes for saving to files etc. Pickable objects are pickable, and recreating an object from a serialized byte sequence is called Unpickle.
By default Pickle tries to serialize the attributes of objects registered in __dict__, but you can customize its behavior by defining a special method for pickle. Especially, it is used when the object to be pickled has an object that cannot be pickled as a member, or when the object is created by a constructor that takes an argument.
| Method | Intended use |
|---|---|
__getnewargs__() |
Specify the argument to be passed to the constructor when unpickling. |
__getstate__() |
__dict__Instead, pickle the object returned by this method. |
__setstate__(state) |
Receives the pickled object as a state and performs the processing at the time of unpickle. |
The following code is a sample script to see the behavior of these methods.
python
import pickle
class Foo:
def __new__(self, a):
print('in __new__:', a)
self = object.__new__(self)
# self.a = a
return self
def __init__(self, a):
print('in __init__:', a)
self.a = a
def __getstate__(self):
print('in __getstate__:', self.a)
state = self.__dict__.copy()
return state
def __getnewargs__(self):
print('in __getnewargs__:', self.a)
return (2 * self.a,)
def __setstate__(self, state):
self.__dict__.update(state)
print('in __setstate__:', self.a)
foo = Foo(3)
bar = pickle.dumps(foo)
baz = pickle.loads(bar)
print(baz.__dict__)
The result of executing this script is as follows.
python
in __new__: 3
in __init__: 3
in __getnewargs__: 3
in __getstate__: 3
in __new__: 6
in __setstate__: 3
{'a': 3}
As stated in the pickle manual, you can see the following.
--__getnewargs__ is called when pickling. In other words, the argument information passed to __new__ when unpickled is pickled.
--The value returned by __getnewargs__ is passed to __new__.
--Unpicle does not call __init__.