This is a conversation that you never want to hear.

"Oh no! The frobnitz is broken!"

"But it was working last week."

"I don't know what happened - was it the changes I made to the gorblewhonker?"

"Wait, I was putting that function in the gorblewhonker. I told you after class on Thursday, remember?"

"Shoot. Do you have last week's version with the working frobnitz? What's the difference?"

"Maybe. I don't know. Does this mean we have to redo all the improvements we've made to the blooglebox since then?"

"Argh."

There are tools that allow you to avoid these kinds of conversations.

Have you ever created a folder that looked something like this?

mycode-1.py
mycode-2.py
mycode-2-with-rachel's-changes.py
mycode-working.py
mycode-LATEST.py

If so, you have used version control. According to Wikipedia version control "is the management of changes to documents, programs, and other information stored as computer files."

A system that manages version control for software development is called a source code management system, or an SCM for short. In this chapter, you will learn the basic use of source control management.