[Answer]-Using Fabric to search for and run a file

Farbic functions allow you to use arguments:

@task   #not bad to use once your fabfile is big by using helper functions
def runserver(project_path, port=8000):
    run("python %s/manage.py runserver 0.0.0.0:%s" % (project_path,port))

and you would use it like this:

fab runserver:/home/project,8080

you could also simplify it by creating a task that selects a port per project, although all available projects and their paths would have to be defined there. Then it could be as easy as:

fab runserver:myprojectname

Of course you could additionally implement @morgan’s answer making the script check if the port is open and automatically assign a free one.