import PySimpleGUI as sg
import sys

if len(sys.argv) == 1:
    fname = sg.Window('My Script',
                    [[sg.Text('Document to open')],
                    [sg.In(), sg.FileBrowse()],
                    [sg.Open(), sg.Cancel()]]).read(close=True)[1][0]
else:
    fname = sys.argv[1]

if not fname:
    sg.popup("Cancel", "No filename supplied")
    raise SystemExit("Cancelling: no filename supplied")
else:
    sg.popup('The filename you chose was', fname)