Pickling Python New Style Object Using State
Posted by sky | Tags: Python
Pickling is basically Python object serialization. See the Python PEP 307 Extensions to the pickle protocol for further details. Below, I show a simple way to pickle a new-style object using the state of the object. obj.__setstate__(state) will be called when unpickling if the state is being pickled.
def __newobj__(cls, *args): return cls.__new__(cls, *args) # NEWOBJ pickle
class AClass(object):
..
def __reduce__(self): return (__newobj__, (self.__class__,), self.__getstate__())
def __getstate__(self):
state = .. # object's data
return state
def __setstate__(self, st):
# restore object's data
When the unpickling function returned by __reduce__ (the first
item of the returned tuple) has the name __newobj__, something
special happens for pickle protocol 2. An unpickling function
named __newobj__ is assumed to have the following semantics:
def __newobj__(cls, *args):
return cls.__new__(cls, *args)
Pickle protocol 2 special-cases an unpickling function with this
name, and emits a pickling opcode that, given 'cls' and 'args',
will return cls.__new__(cls, *args) without also pickling a
reference to __newobj__ (this is the same pickling opcode used by
protocol 2 for a new-style class instance when no __reduce__
implementation exists). This is the main reason why protocol 2
pickles are much smaller than classic pickles.