 What's good opensource.com? Justin Florey here again for this week's top five articles of the week for April 7th. As this week comes to a close, let's check out some of this week's hottest articles. Topics for this week include mocking web APIs with flashback, the secret, or not so secret, role of sustainers in open source, validating certificates with Golang, building GUIs with Python frameworks, and a fly-by introduction to functional programming. Coming in at number five, introducing flashback, an internet mocking tool. Automatic tests are only as useful as they are reliable. Fang, a first-time writer to opensource.com, introduces how LinkedIn uses flashback for mocking HTTP and HTTPS resources like web services and REST APIs for testing purposes. Learn how you can use it to do the same. Coming in at number four, why you should become an open source sustainer. Most people in open source understand what maintainers and contributors are, but what about sustainers? Justin Dorfman introduces the hidden role of a third group. The open source sustainers and how advocacy and support are vital to sustain open source communities. At number three, Golang to the rescue, saving DevOps from TLS turmoil. Chris Short had a problem. He needed new certificates and certificate chains for his company's site. However, before deploying them, he wanted to have testing and validation before making any changes. Check out his simple solution written in Golang that does just this and see how you can do the same. At number two, QT versus WX. How do two of the most popular Python frameworks compare? Python is a widely known programming language and has different frameworks for creating graphical user interfaces or GUIs. Seth Kinlan introduces WX Python and PyQT and helps teach readers which ones best suit their needs. And finally, coming in at the number one spot, introduction to functional programming. What is functional programming? What are the benefits and how can you learn more? Fraser Tweedale breaks down the basics, takes on some myths and gives some pointers on what you can do to brush up your skills. Catch you next week, opensource.com.