Programming Python, 3rd Edition

Pickled Objects

Probably the biggest limitation of DBM keyed files is in what they can store: data stored under a key must be a simple text string. If you want to store Python objects in a DBM file, you can sometimes manually convert them to and from strings on writes and reads (e.g., with str and eval calls), but this takes you only so far. For arbitrarily complex Python objects such as class instances and nested data structures, you need something more. Class instance objects, for example, cannot be later re-created from their standard string representations. Custom to-string conversions are error prone and not general.

The Python pickle module, a standard part of the Python system, provides the conversion step needed. It converts nearly arbitrary Python in-memory objects to and from a single linear string format, suitable for storing in flat files, shipping across network sockets between trusted sources, and so on. This conversion from object to string is often called serialization—arbitrary data structures in memory are mapped to a serial string form.

The string representation used for objects is also sometimes referred to as a byte stream, due to its linear format. It retains all the content and references structure of the original in-memory object. When the object is later re-created from its byte string, it will be a new in-memory object identical in structure and value to the original, though located at a different memory address. The re-created object is effectively ...