Hello Alessio,
If you mean a session file, where you can open this file later in ChimeraX to restore your work, that file type is .cxs --- although Chimera had a .py session file, ChimeraX does not, it has .cxs instead.  E.g.  command

save myfile.cxs

...or use menu: File... Save and choose Files of type: ChimeraX session.  Later you can start ChimeraX and then open the .cxs file to restore your work, with the "open" command or menu: File... Open.

Session file information:
<https://rbvi.ucsf.edu/chimerax/docs/user/commands/save.html#session>

I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Resource for Biocomputing, Visualization, and Informatics
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Aug 12, 2024, at 6:29 AM, Alessio Nocentini via ChimeraX-users <chimerax-users@cgl.ucsf.edu> wrote:
>
> Dear Elaine,
>
> I need your support, as I do not understand how to save a project .py file in chimeraX. I tried to follow the instructions, but did not find it.
>
> Thank you
>
> Best
>
> A