Objects in dictionaries can be accessed by keys. There is a similar way to access particular data in a file by first assigning it a key. This is possible by using the module shelve:

from contextlib import closing
import shelve as sv
# opens a data file (creates it before if necessary)
with closing(sv.open('datafile')) as data:
    A = array([[1,2,3],[4,5,6]])     
    data['my_matrix'] = A  # here we created a key

In the section File handling, we saw that the built-in open command generates a context manager, and we saw why this is important for handling external resources, such as files. In contrast to this command, sv.open does not create a context manager by itself. The closing command from the contextlib module is needed to transform it into an appropriate context manager. Consider the following example of restoring the file:

from contextlib import closing
import shelve as sv
with closing(sv.open('datafile')) as data: # opens a data file
    A = data['my_matrix']  # here we used the key
    ...

A shelve object has all dictionary methods, for example, keys and values, and can be used in the same way as a dictionary. Note that changes are only written in the file after the close or sync method has been called.