Python Cookbook

Mutating Objects with shelve

Credit: Luther Blissett

Problem

You are using the standard module shelve, some of the values you have shelved are mutable objects, and you need to mutate these objects.

Solution

The shelve module, which offers a kind of persistent dictionary, occupies an important niche between the power of relational-database engines and the simplicity of marshal, pickle, dbm, and similar file formats. However, there’s a typical trap that you need to avoid when using shelve. Consider the following:

>>> import shelve
>>> # Build a simple sample shelf
>>> she=shelve.open('try.she', 'c')
>>> for c in 'spam': she[c]={c:23}
...
>>> for c in she.keys(  ): print c, she[c]
...
p {'p': 23}
s {'s': 23}
a {'a': 23}
m {'m': 23}
>>> she.close(  )

We’ve created the shelve object, added some data to it, and closed it. Now we can reopen it and work with it:

>>> she=shelve.open('try.she','c')
>>> she['p']
{'p': 23}
>>> she['p']['p'] = 42
>>> she['p']
{'p': 23}

What’s going on here? We just set the value to 42, but it didn’t take in the shelve object. The problem is that we were working with a temporary object that shelve gave us, but shelve doesn’t track changes to the temporary object. The solution is to bind a name to this temporary object, do our mutation, and then assign the mutated object back to the appropriate item of shelve:

>>> a = she['p']
>>> a['p'] = 42
>>> she['p'] = a
>>> she['p']
{'p': 42}
>>> she.close(  )

We can even verify the change:

>>> she=shelve.open('try.she','c') >>> for ...