Storing Objects on a Shelve Database

Pickling and shelves are somewhat advanced topics, and we won’t go into all their details here; you can read more about them in the standard library manuals, as well as application-focused books such as Programming Python. This is all simpler in Python than in English, though, so let’s jump into some code.

Let’s write a new script that throws objects of our classes onto a shelve. In your text editor, open a new file we’ll call makedb.py. Since this is a new file, we’ll need to import our classes in order to create a few instances to store. We used from to load a class at the interactive prompt earlier, but really, as with functions and other variables, there are two ways to load a class from a file (class names are variables like any other, and not at all magic in this context):

广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元

import person                                   # Load class with import
bob = person.Person(...)                        # Go through module name

from person import Person                       # Load class with from
bob = Person(...)                               # Use name directly

We’ll use from to load in our script, just because it’s a bit less to type. Copy or retype this code to make instances of our classes in the new script, so we have something to store (this is a simple demo, so we won’t worry about the test-code redundancy here). Once we have some instances, it’s almost trivial to store them on a shelve. We simply import the shelve module, open a new shelve with an external filename, assign the objects to keys in the shelve, and close the shelve when we’re done because we’ve made changes:

# File makedb.py: store Person objects on a shelve database

from person import Person, Manager              # Load our classes
bob = Person('Bob Smith')                       # Re-create objects to be stored
sue = Person('Sue Jones', job='dev', pay=100000)
tom = Manager('Tom Jones', 50000)

import shelve
db = shelve.open('persondb')                    # Filename where objects are stored
for object in (bob, sue, tom):                  # Use object's name attr as key
    db[object.name] = object                    # Store object on shelve by key
db.close()                                      # Close after making changes

Notice how we assign objects to the shelve using their own names as keys. This is just for convenience; in a shelve, the key can be any string, including one we might create to be unique using tools such as process IDs and timestamps (available in the os and time standard library modules). The only rule is that the keys must be strings and should be unique, since we can store just one object per key (though that object can be a list or dictionary containing many objects). The values we store under keys, though, can be Python objects of almost any sort: built-in types like strings, lists, and dictionaries, as well as user-defined class instances, and nested combinations of all of these.

That’s all there is to it—if this script has no output when run, it means it probably worked; we’re not printing anything, just creating and storing objects:

C:\misc> makedb.py