rework the decision tree for interactive mode

crawl.py

@@ -82,20 +82,37 @@ d | date: Print the date which the data is from.
             print(res.date)
         elif len(argv[0]) == 3 or len(argv[1]) == 3:
             # more than 3 letter abbreviations are invalid
-            if is_float(argv[0]):
+            cur_use = str
-                # amount first -> convert to currency in argv[1]
+            amt_use = float
-                print(fmt_and_calc(
+            dir_use = int
-                    cur=argv[1].upper(),
+            if len(argv) == 1 and not is_float(argv[0]):
-                    amt=float(argv[0]),
+                # only currency specified
-                    res=res,
+                cur_use = argv[0].upper()
-                    direction=DIRECTION_FROM_EUR))
+                amt_use = 1
+                dir_use = DIRECTION_TO_EUR
+            elif is_float(argv[0]):
+                # amount first -> convert EUR to currency in argv[1]
+                cur_use = argv[1].upper()
+                amt_use = float(argv[0])
+                dir_use = DIRECTION_FROM_EUR
             elif is_float(argv[1]):
-                # currency first -> convert to EUR
+                # currency first -> convert argv[1] to EUR
+                cur_use = argv[0].upper()
+                amt_use = float(argv[1])
+                dir_use = DIRECTION_TO_EUR
+            else:
+                raise ValueError
+            
+            # check that the currency exists
+            if cur_use not in res.rates:
+                raise ValueError
+            else:
                 print(fmt_and_calc(
-                    cur=argv[0].upper(),
+                    cur=cur_use,
-                    amt=float(argv[1]),
+                    amt=amt_use,
                     res=res,
-                    direction=DIRECTION_TO_EUR))
+                    direction=dir_use
+                ))
         else:
             print("Not implemented: '" + " ".join(argv) + "'")
     except IndexError:
@@ -103,6 +120,8 @@ d | date: Print the date which the data is from.
             pass
         else:
             print("Too few arguments: '" + " ".join(argv) + "'")
+    except ValueError:
+        print("The currency specified does not exist.")
 
 def is_float(string: str) -> bool:
     try: