The biggest difference that stuck out in switching was the way PHP was effectively like markup language, while Python usually involves its own web server. This can be a difficult transition because instead of thinking in terms of requests to specific files, you have the option to handle all requests however you'd like. It allows for more flexibility, but you usually have a more difficult time with deployment since things like static files are better served with a traditional web server. Likewise, things like different applications need a little more thought than simply uploading a file. That said, there is a wealth of great libraries and the standard library that help not only provide functionality, but help promote better code.