Add usage

cheapest_ticket.py

@@ -5,11 +5,27 @@ import sys
 
 import captaintrain as ct
 
+def usage():
+    print("""cheapest_ticket.py departure-station arrival-station departure-date
+
+    departure-date: any string parsable by date -d
+""")
 
 if __name__ == "__main__":
-    if len(sys.argv) > 3:
+    if len(sys.argv) <= 3:
+        usage()
+        sys.exit(1)
+
     departure = ct.station(sys.argv[1])
+    if departure is None:
+        print("Unknown station '%s'" % sys.argv[1])
+        sys.exit(1)
+
     arrival = ct.station(sys.argv[2])
+    if arrival is None:
+        print("Unknown station '%s'" % sys.argv[2])
+        sys.exit(1)
+
     departure_time = 0
 
     with subprocess.Popen(["date", "-d", sys.argv[3], "-u", "-Iseconds"], stdout=subprocess.PIPE) as f:
@@ -32,5 +48,3 @@ if __name__ == "__main__":
     min_trips = ct.cheapest_trips(res.trips)
     for trip in min_trips:
         print(trip)
-    else:
-        print("usage")