Home > Articles > Programming > Python

  • Print
  • + Share This
Like this article? We recommend Glue Logic: A Sticky Business

Glue Logic: A Sticky Business

In this context, glue logic is simply a piece of Python script that allows you to call your legacy C++ programs, with no need to change them. Suppose we have a bunch of such C++ programs that must be called from Python, with the major requirement that we cannot make any changes to the C++ code. In other words, C++ code exists and is in use by real users. We can call it—but not change it!

With this requirement in mind, let's look at an example of the glue logic approach.

Calling C++ Programs from Python

Let's say we have a really simple C++ program (called CPlusPlus0xProject) that we want to call from Python. Listing 1 illustrates an excerpt from the C++ program.

Listing 1—A simple C++ program.

int main() {
        cout << endl << "Now calling doSomeMemoryWork()" << endl;
        cout << "Now calling getPersonDetails()" << endl;

        return 0;

How do we run this C++ program from Python? This goal is achieved with the Python script in Listing 2.

Listing 2—Python invocation of a C++ program.

import os, errno
import subprocess as sp

def invokeProgram():
        print('Just before Python program invocation')
        p = sp.Popen(['/home/stephen/CPlusPlus0xProject'], stdout = sp.PIPE)
        result = p.communicate()[0]
        print('CPlusPlus0xProject invoked'  + result + '\n')
    except OSError as e:
        print('Exception handler error occurred: '  + str(e.errno) + '\n')

if __name__ == '__main__':

In Listing 2, our simple C++ program is invoked from Python by using the subprocess module. The script imports a few standard modules, defines a function, and then has a main section. The C++ program can be anything you like; Listing 2 is essentially the same as running the C++ program directly from the console.

Listing 3 shows the combined Python and C++ program output.

Listing 3—The Python and C++ program output.

Just before Python program invocation
CPlusPlus0xProject invoked
Now calling doSomeMemoryWork()

Function name: doSomeMemoryWork()
Before memory assignment
a = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
b = bbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
c = cccccccccccccccccccccccccccccc
After memory assignment
a = bbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
b = bbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
c = cccccccccccccccccccccccccccccc
Now calling getPersonDetails()
Person just created is: John Doe
Person name = John Doe
Now inside the Person class destructor

Why go to the trouble of using Python like this? Isn't it just as easy to run the C++ program from the console?

Well, using Python like this gives you a great deal more power than is the case with the console. For one thing, you can take advantage of Python's exception-management facilities—this is the try-except block in Listing 2. If anything goes wrong inside the Python exception block, you'll see a message printed on the console. For example, let's change this line in Listing 2:

p = sp.Popen(['/home/stephen/CPlusPlus0xProject'], stdout = sp.PIPE)

to this:

p = sp.Popen(['./home/stephen/CPlusPlus0xProject'], stdout = sp.PIPE)

Running this erroneous Python code results in the following program output:

Just before Python program invocation
Exception handler error occurred: 2

Notice the exception that has been caught and reported. Now, you might be thinking, "This fellow is very pessimistic in outlook, worrying about exceptions from the word 'go.'" This is a good point, but exceptions are generally an unavoidable fact of life. Exception handling is perhaps not the most glamorous of areas to code, but working hard on your exception logic makes for a more solid end product. This in turn provides for an improved end-user experience.

Clearly, the code being executed in these examples is very simple. But as you scale up the size of both the Python code and the C++ programs, you'll begin to see the power of the glue logic approach. A large number of C++ programs can be invoked from a fairly simple Python script.

Now let's take a closer look at Python exceptions.

Python Exceptions

Python has a powerful exception-management facility and is substantially documented online. Listing 4 shows a typical exception example.

Listing 4—Python I/O exception management.

    fh = open('/home/stephen/myFile.txt', "w")
    fh.write("A test file.")
except IOError as e:
    print('Exception handler error occurred: '  + str(e.errno) + '\n')
    print 'Successfully updated file.'

The script in Listing 4 opens a file in write mode and then attempts to write to the file. If no errors occur, the output should be that associated with the else clause:

Successfully updated file.

If an IOError exception occurs, the except clause is executed.

The Python exception code we've seen so far is very similar to the equivalent Java (or even C++) mechanism. The major merit of such exception handling is that errors can be handled close to where they occur. If the code in question can't handle the exception, at the very least it should document the exception in a logfile and then rethrow it. The latter scenario then gives higher-level code (that is, the calling code) a chance to handle the exception. Think of this as the rule:

Exceptions are inconvenient, but they should never be ignored or swallowed by code without a really good reason.

Given that Python has strong exception-support facilities, is it a full development language?

  • + 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.


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