The pickle Module

The pickle module, shown in Example 4-11, is used to serialize datathat is, convert data to and from character strings, so that they can be stored on file or sent over a network. It's a bit slower than marshal, but it can handle class instances, shared elements, and recursive data structures, among other things.

Example 4-11. Using the pickle Module

File: pickle-example-1.py import pickle value = ( "this is a string", [1, 2, 3, 4], ("more tuples", 1.0, 2.3, 4.5), "this is yet another string" ) data = pickle.dumps(value) # intermediate format print type(data), len(data) print "-"*50 print data print "-"*50 print pickle.loads(data) 121 -------------------------------------------------- (S'this is a string' p0 (lp1 I1 aI2 aI3 aI4 a(S'more tuples' p2 F1.0 F2.3 F4.5 tp3 S'this is yet another string' p4 tp5 . -------------------------------------------------- ('this is a string', [1, 2, 3, 4], ('more tuples', 1.0, 2.3, 4.5), 'this is yet another string')

On the other hand, pickle cannot handle code objects (but see the copy_reg module for a way to fix this).

By default, pickle uses a text-based format. You can also use a binary format, in which numbers and binary strings are stored in a compact binary format. The binary format usually results in smaller files. This is demonstrated in Example 4-12.

Example 4-12. Using the pickle Module in Binary Mode

File: pickle-example-2.py import pickle import math value = ( "this is a long string" * 100, [1.2345678, 2.3456789, 3.4567890] * 100 ) # text mode data = pickle.dumps(value) print type(data), len(data), pickle.loads(data) == value # binary mode data = pickle.dumps(value, 1) print type(data), len(data), pickle.loads(data) == value