Hello, @Cyrille! Suppose you have some code written in a file called "my_code.py", and you want to use it in a Sage Notebook. All you have to do is use the command load("my_code.py").
Of course, this example assumes that you have created a notebook in the same folder as the file containing your code. If you have it some where else, you can specify its complete location as the argument of load. For example, suppose you you have your file in your home directory in Linux, you can write in your notebook the following:
load("/home/cyrille/my_code.py")

From the point where you use this command, you will be able to use the functions and subroutines defined in my_code.py.