Because you don&#x27;t know what libraries you may depend on in the future when you start a new project. I was a fervent Python 3 supporter, and wrote every of my fresh projects in Python 3 instead of 2, until one day I found I needed LLVM in my project, yet the python port at that time was for 2 only.<p>I mostly avoid writing Python 3 nowadays, because I don&#x27;t want to rewrite my project or find painstakingly an alternative solution when I could have just imported a module that runs fine under Python 2.