def pi_changehostname(oldname,newname):
# hostname
with fileinput.FileInput("/etc/hostname", inplace=True, backup='.bak') as file:
    for line in file:
        print(line.replace(oldname,newname), end='')
        if oldname in line:
            break
# hosts
with fileinput.FileInput("/etc/hosts", inplace=True, backup='.bak') as file:
    for line in file:
        print(line.replace(oldname,newname), end='')
        if oldname in line:
            break