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Saving a Python dict to a file using pickle

Date: 2008-01-07  |   Modified: 2008-09-09  |   Tags: datastructures, python   |   4 Comments

Per Programming Python, 3rd Edition, there are a number of methods to store persistent data with Python:

  • I often use flat files to read or write text (string) data using the os library.
  • Flat files are read sequentially, but dbm files allow for keyed access to string data
  • The pickle module can be used to store non-string Python data structures, such as Python dicts. However, the data is not keyed as with dbm files.
  • shelve files combine the best of the dbm and pickle methods by storing pickled objects in dbm keyed files.
  • I've read good things about the ZODB object-oriented database , but I don't know too much about it. Per the book, it is a more powerful alternative to shelves.
  • The final option is interfacing with a full-fledged SQL relational databases. As I mentioned before, Python 2.5 has an interface to SQLite as part of the standard distribution.

Here is an example using pickle which writes a Python dict to a file and reads it back again:

import pickle

# write python dict to a file
mydict = {'a': 1, 'b': 2, 'c': 3}
output = open('myfile.pkl', 'wb')
pickle.dump(mydict, output)
output.close()

# read python dict back from the file
pkl_file = open('myfile.pkl', 'rb')
mydict2 = pickle.load(pkl_file)
pkl_file.close()

print mydict
print mydict2

Results: 
{'a': 1, 'c': 3, 'b': 2}
{'a': 1, 'c': 3, 'b': 2}

4 Comments — feed icon Comments feed for this post


#1 code43 commented on 2009-11-18:

hi Eliot, here's an appropriate Python module for you:

http://yserial.sourceforge.net

The module is instructive in the way it unifies the standard batteries: sqlite3 (as of Python v2.5), zlib (for compression), and cPickle (for securely serializing objects).

If your Python program requires data persistance, then y_serial is a module which should be worth importing. Objects are warehoused in a database file in very compressed form. Steps for insertion, organization by annotation, and finally retrieval are amazingly simple.

Hope this helps...


#2 Eliot commented on 2009-11-18:

code43, looks good, thanks!


#3 __django__ commented on 2010-05-26:

Thanks for code. I looking for.


#4 PATX commented on 2010-06-29:

I always refer back to this post every time I do something with dicts... Well most of the time :P. I'm forgetful and this is super helpful.

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