You are here


8 September, 2015 - 10:52

A limitation of anydbm is that the keys and values have to be strings. If you try to use any other type, you get an error.

The pickle module can help. It translates almost any type of object into a string suitable for storage in a database, and then translates strings back into objects.

pickle.dumps takes an object as a parameter and returns a string representation (dumps is short for “dump string”):

>>> import pickle>>> t = [1, 2, 3]>>> pickle.dumps(t)'(lp0\nI1\naI2\naI3\na.'

The format isn’t obvious to human readers; it is meant to be easy for pickle to interpret. pickle.loads (“load string”) reconstitutes the object:

>>> t1 = [1, 2, 3]>>> s = pickle.dumps(t1)>>> t2 = pickle.loads(s)>>> print t2[1, 2, 3]

Although the new object has the same value as the old, it is not (in general) the same object:

>>> t1 == t2True>>> t1 is t2False

In other words, pickling and then unpickling has the same effect as copying the object.

You can use pickle to store non-strings in a database. In fact, this combination is so common that it has been encapsulated in a module called shelve.

Exercise 14.3.If you download my solution to Exercise 12.4 in Exercises from http: // thinkpython. com/code/ anagram_ sets. py, you’ll see that it creates a dictionary that maps from a sorted string ofletters to the list of words that can be spelled with those letters. For example,'opst'maps to thelist['opts', 'post', 'pots', 'spot', 'stop', 'tops'].

Write a module that importsanagram_setsand provides two new functions:store_anagramsshould store the anagram dictionary in a “shelf;”read_anagramsshould look up a word and returna list of its anagrams. Solution: http: // thinkpython. com/ code/ anagram_ db. py