Pickle, convert an object to a byte stream

Unpickle, construct the object from the byte stream

Unpickle process is not safe. It is possible to construct malicious pickle data which will execute arbitrary code during unpickling

non-Python programs may not be able to reconstruct pickled Python objects

cPickle has been removed in Python 3

• None, True, and False
• integers, long integers, floating point numbers, complex numbers
• normal and Unicode strings
• tuples, lists, sets, and dictionaries containing only picklable objects
• functions defined at the top level of a module
• built-in functions defined at the top level of a module
• classes that are defined at the top level of a module
• instances of such classes whose __dict__ or the result of calling __getstate__() is picklable (see section The pickle protocol for details).

0, ASCII protocol

1, old binary format

2, python 2.3

3, python 3.0

4, python 3.4

5, python 3.8

-1, highest available protocol

The higher the protocol used, the more recent the version of Python needed to read the pickle produced

print(pickle.HIGHEST_PROTOCOL) # highest protocol
print(pickle.DEFAULT_PROTOCOL) # default protocol
        

# write pickled object to file
import pickle
 
f = open('temp.pkl', 'wb')
 
l = list(range(10))
 
pickle.dump(l, f) # default protocol
# pickle.dump(l, f, 5)
 
f.close();
        

# read pickled object from file and unpickle
f = open('temp.pkl', 'rb')

l = pickle.load(f)
print(l)

f.close()
		

l = list(range(10))
 
# write pickled object to bytes
s = pickle.dumps(l, 2)
print(s)
 
# read pickled object from bytes and unpickle
t = pickle.loads(s)
print(t)
		

f = open('temp.pkl', 'wb')
l = list(range(10))

# pickle
p_write = pickle.Pickler(f)
p_write.dump(l)

f.close()
        

f = open('temp.pkl', 'rb')
p_read = pickle.Unpickler(f)

# unpickle
print(p_read.load())

f.close()
        

__getstate__

• if the class defines the method __getstate__(), it is called and then return state is pickled as the contents for the instance
• if there is no __getstate__() method, the instance's __dict__ is pickled

__setstate__

• if the class also defines the method __setstate__(), it is called with the unpickled state
• if there is no __setstate__() method, the pickled state must be a dictionary and its items are assigned to the new instance's dictionary

class Vehicle(object):
 
    def __init__(self, brand):
        self.__brand = brand
        
    def __str__(self):
        return "Info: %s" % self.__dict__
 
    def __getstate__(self):
        print('Call __getstate__ for pickling ...')
        state = self.__dict__.copy()
        return state
 
    def __setstate__(self, state):
        print('Call __setstate__ for unpickling ...')
        self.__dict__.update(state)
        

# pickle object
f = open('temp.pkl', 'wb')
pickle.dump(v, f) # call __getstate__()
f.close()
        

# unpickle object
f = open('temp.pkl', 'rb')
l = pickle.load(f) # call __setstate__()
print(l) # call __str__()
f.close()
        

• pickle — Python object serialization