11. Data Persistence

The modules described in this chapter support storing Python data in a persistent form on disk. The pickle and marshal modules can turn many Python data types into a stream of bytes and then recreate the objects from the bytes. The various DBM-related modules support a family of hash-based file formats that store a mapping of strings to other strings. The bsddb module also provides such disk-based string-to-string mappings based on hashing, and also supports B-Tree and record-based formats.

The list of modules described in this chapter is:

• pickle — Python object serialization * Relationship to other Python modules * Data stream format * Usage * What can be pickled and unpickled? * The pickle protocol

  • Pickling and unpickling normal class instances
  • Pickling and unpickling extension types
  • Pickling and unpickling external objects

  • Subclassing Unpicklers
  • Example

• cPickle — A faster pickle

• copy_reg — Register pickle support functions

• shelve — Python object persistence * Restrictions * Example

• marshal — Internal Python object serialization

• anydbm — Generic access to DBM-style databases

• whichdb — Guess which DBM module created a database

• dumbdbm — Portable DBM implementation * Dumbdbm Objects