3 months is the standard notice time for a developer (and for most qualified jobs) in France. This is written in your contract when you get a job. This is both ways: when you leave or when you are fired.
So leaving without notice time (or with a shorter time) is something you have to negociate.
Now that is perfectly fine. The standard of professionalism applies equally to bother employer and employee. I have absolutely no problem with that.