Exclude object's field from pickling in python
One way to handle instance attributes that are not picklable objects is to use the special methods available for modifying a class instance's state: __getstate__()
and __setstate__()
. Here is an example
class Foo(object):
def __init__(self, value, filename):
self.value = value
self.logfile = file(filename, 'w')
def __getstate__(self):
"""Return state values to be pickled."""
f = self.logfile
return (self.value, f.name, f.tell())
def __setstate__(self, state):
"""Restore state from the unpickled state values."""
self.value, name, position = state
f = file(name, 'w')
f.seek(position)
self.logfile = f
When an instance of Foo
is pickled, Python will pickle only the values returned to it when it calls the instance's __getstate__()
method. Likewise, during unpickling, Python will supply the unpickled values as an argument to the instance's __setstate__()
method. Inside the __setstate__()
method we are able to recreate the file object based on the name and position information we pickled, and assign the file object to the instance's logfile attribute.
Reference: http://www.ibm.com/developerworks/library/l-pypers.html
There is an example in the docs which solves your problem with __getstate__
and __setstate__
.
Pickling uses the object's __getstate__
and __setstate__
methods; you can override them and ignore the fields you want.
# foo.py
class Foo:
def __init__(self):
self.bar = 1
self.baz = 2
def __getstate__(self):
state = self.__dict__.copy()
# Don't pickle baz
del state["baz"]
return state
def __setstate__(self, state):
self.__dict__.update(state)
# Add baz back since it doesn't exist in the pickle
self.baz = 0
# main.py
import pickle
from foo import Foo
foo = Foo()
print(f"Foo bar: {foo.bar} baz: {foo.baz}")
new_foo = pickle.loads(pickle.dumps(foo))
print(f"New bar: {new_foo.bar} baz: {new_foo.baz}")
Output:
Foo bar: 1 baz: 2
New bar: 1 baz: 0
You can find another example here: https://docs.python.org/3/library/pickle.html#handling-stateful-objects