Home > Articles > Web Development

This chapter is from the book

Item 4: Be Aware That Constants Are Mutable

If you’re coming to Ruby from another programming language, there’s a good chance that constants don’t behave the way you expect them to. But before we dig into that let’s review what Ruby considers to be a constant.

When you first learned Ruby you were probably taught that constants are identifiers that are made up of uppercase alphanumeric characters and underscores. Some examples include STDIN, ARGV, and RUBY_VERSION. But that’s not the entire story. In reality, a constant is any identifier that begins with an uppercase letter. This means that identifiers like String and Array are also constants. That’s right...the names of classes and modules are actually constants in Ruby. With that in mind, let’s take a closer look at how constants differ from other variable-like things in Ruby.

As their name suggests, constants are meant to remain unchanged during the lifetime of a program. You might assume, therefore, that Ruby would prevent you from altering the value stored in a constant. Well, that assumption would be wrong. Consider this:

module Defaults
  NETWORKS = ["192.168.1", "192.168.2"]
def purge_unreachable (networks=Defaults::NETWORKS)
  networks.delete_if do |net|
    !ping(net + ".1")

If you invoke the purge_unreachable method without an argument, it will accidentally mutate a constant. It will do this without so much as a warning from Ruby. Essentially, constants are more like global variables than unchanging values. If you think about it, since class and module names are constants, and you can change a class at anytime (e.g., add methods), then the objects referenced by constants need to be mutable in Ruby. That’s fine for classes and modules, but not so great for the values we actually want to be constant and immutable. Thankfully, there’s a solution to this problem—the freeze method:

module Defaults
  NETWORKS = ["192.168.1", "192.168.2"].freeze

With this change in place, the purge_unreachable method will raise a RuntimeError exception if it tries to alter the array referenced by the NETWORKS constant. As a general rule of thumb, always freeze constants to prevent them from being mutated. Unfortunately, freezing the NETWORKS array isn’t quite enough. Consider this:

def host_addresses (host, networks=Defaults::NETWORKS)
  networks.map {|net| net << ".#{host}"}

The host_addresses method will modify the elements of the NETWORKS array if it isn’t given a second argument. While the NETWORKS array itself is frozen, its elements are still mutable. You might not be able to add or remove elements from the array, but you can surely make changes to the existing elements. So, if a constant references a collection object such as an array or hash, freeze the collection and its elements:

module Defaults

(If you happen to be using Ruby 2.1 or later you can make use of a trick from Item 47 and freeze the string literals directly. This can save you a bit of memory while keeping the elements from accidentally being mutated.)

Freezing a constant will change an obscure, hard-to-track-down bug into an exception. That’s an obvious win. Unfortunately, it’s still not enough. Even if you freeze the object a constant refers to, you can still cause problems by assigning a new value to an existing constant. See for yourself:

irb> TIMEOUT = 5
---> 5

irb> TIMEOUT += 5
(irb):2: warning: already initialized constant TIMEOUT
(irb):1: warning: previous definition of TIMEOUT was here
---> 10

As you can see, assigning a new value to an existing constant is perfectly legal in Ruby. You can also see that Ruby produces a warning telling us that we’re redefining a constant. But that’s it, just a warning. Thankfully, if we take things into our own hands, we can make Ruby raise an exception if we accidentally redefine a constant. The solution is a bit clumsy, and may be too heavy-handed for some situations, but it’s simple. To prevent Ruby from assigning new values to existing constants, freeze the class or module they’re defined in. You may even want to structure your code so that all constants are defined in their own module, isolating the effects of the freeze method:

module Defaults


There are three levels of freezing you should consider when defining constants. The first two are easy: freeze the object that the constant references and the module the constant is defined in. Those two steps prevent the constant from being mutated or assigned to. The third is a bit more complicated. We saw that if a constant references an array of strings, we need to freeze the array and the elements. In other words, you need to deeply freeze the object the constant refers to. Each constant will be different, just make sure it’s completely frozen.

Things to Remember

  • Always freeze constants to prevent them from being mutated.
  • If a constant references a collection object such as an array or hash, freeze the collection and its elements.
  • To prevent assignment of new values to existing constants, freeze the module they’re defined in.

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.


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.


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.


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.


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


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


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.


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.


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