Home > Articles > Programming > Python

ZODB for Python Programmers

  • Print
  • + Share This
This article covers the basics of the Zope Object Database and its use with the Python programming language.
This article is excerpted from The Zope Book, by Michael Pelletier and Amos Latteier.
Like this article? We recommend

Like this article? We recommend

Zope Object Database (ZODB) is a powerful object database for Python objects that comes with Zope (http://www.zope.org). If you've ever worked with a relational database, such as PostgreSQL, MySQL, or Oracle, then you should be familiar with the role of a database. If not, just start by thinking of it as short or long-term storage for your application data.

For many tasks, relational databases are clearly a good solution, but sometimes relational databases don't fit well with your object model. If you have lots of different kinds of interconnected objects with complex relationships and changing schemas then ZODB might be worth giving a try.

A major feature of ZODB is transparency. You do not need to write any code to explicitly read or write your objects to or from a database. Just put your persistent objects into a container that works just like a Python dictionary. Everything inside this dictionary is saved in the database. This dictionary is said to be the root of the database. It's like a magic bag; any Python object that you put inside it becomes persistent.

Actually, there are a few restrictions on what you can store in the ZODB. You can store any objects that can be pickled into a standard, cross-platform serial format. Objects such as lists, dictionaries, and numbers can be pickled. Objects such as files, sockets, and Python code objects, cannot be stored in the database because they cannot be pickled. For more information on pickling, see the Python pickle module documentation (http://www.python.org/doc/current/lib/module-pickle.html).

A Simple Example

The first thing you need to do to start working with ZODB is to create a root object. This process involves first opening a connection to a storage, which is the actual back-end that stores your data.

ZODB supports many pluggable storage back-ends, but for the purposes of this article I'm going to show you how to use the FileStorage back-end storage, which stores your object data in a file. Other storages include storing objects in relational databases, Berkeley databases, and a client-to-server storage, which stores objects on a remote storage server.

To set up a ZODB, you must first install it. ZODB comes with Zope, so the easiest way to install ZODB is to install Zope and use the ZODB that comes with your Zope installation. For those of you who don't want all of Zope, but want ZODB, see the instructions for downloading StandaloneZODB from the ZODB (http://www.zope.org/Wikis/ZODB/FrontPage) web page.

StandaloneZODB can be installed into your system's Python libraries using the standard distutils Python module.

After installing ZODB, you can start to experiment with it right from the Python command-line interpreter. For example, try the following Python code in your interpreter:

    >>> from ZODB import FileStorage, DB
>>> storage =
FileStorage.FileStorage('mydatabase.fs')
>>> db = DB(storage)
>>> connection = db.open()
>>> root = connection.root()

Here, you create storage and use the mydatabse.fs file to store the object information. Then, you create a database that uses that storage.

Next, the database needs to be opened by calling the open() method. This returns a connection object to the database. The connection object then gives you access to the root of the database with the root() method.

The root object is the dictionary that holds all of your persistent objects. For example, you can store a simple list of strings in the root object:

   >>> root['employees'] = ['Mary', 'Jo', 'Bob']

Now, you have changed the persistent database by adding a new object, but this change so far is only temporary. To make the change permanent you must commit the current transaction:

   >>> get_transaction().commit()

Transactions group lots of changes into one atomic operation. In a later article, I'll show you how this is a very powerful feature. For now, you can think of committing transactions as checkpoints where you save the changes you've made to your objects so far. Later on, I'll show you how to abort those changes, and how to undo them after they are committed.

Now let's find out if our data was actually saved. First close the database connection:

   >>> connection.close()

Then quit Python. Now start the Python interpreter up again, and connect to the database you just created:

    >>> from ZODB import FileStorage, DB
>>> storage =
FileStorage.FileStorage('mydatabase.fs')
>>> db = DB(storage)
>>> connection = db.open()
>>> root = connection.root()

Now, let's see what's in the root:

>>> root.items() [('employees', ['Mary', 'Jo', 'Bob'])]

There's your list. If you had used a relational database, you would have had to issue a SQL query to save even a simple Python list, such as the preceding example. You would have also needed some code to convert a SQL query back into the list when you wanted to use it again. You don't have to do any of this work when using ZODB. Using ZODB is almost completely transparent; in fact, ZODB-based programs often look suspiciously simple!

Keep in mind that ZODB's persistent dictionary is just the tip of the persistence iceberg. Persistent objects can have attributes that are themselves persistent. In other words, even though you might have only one or two top-level persistent objects as values in the persistent dictionary, you can still have thousands of subobjects lower than them. This is, in fact, how Zope does it. In Zope, only one top-level object exists, which is the root application object for all other objects in Zope.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020