Add mutually exclusive arguments --file and --all

Only one of these arguments must be supplied, or argparse raises an error.

lang/update-po.py

@@ -7,7 +7,8 @@ by calling within the ./po folder:
 python ../lang-extract.py --no-missing -s -o Firmware.pot ../../Firmware/[a-zA-Z]*.[ch]* ../../Firmware/mmu2/[a-zA-Z]*.[ch]*
 """
 
-import sys
+import argparse
+from sys import stderr, exit
 from pathlib import Path
 import polib
 from polib import POFile
@@ -19,7 +20,21 @@ POT_REFERENCE: POFile = polib.pofile(PO_DIR/'Firmware.pot')
 
 
 def main():
-    PO_FILE_LIST = sorted(PO_DIR.glob('**/*.po'))
+    global PO_FILE_LIST
+    ap = argparse.ArgumentParser()
+    group = ap.add_mutually_exclusive_group(required=True)
+    group.add_argument('-f', '--file', help='File path for a single PO file to update Example: ./po/Firmware_cs.po')
+    group.add_argument('--all', action='store_true', help='Update all PO files at once')
+    args = ap.parse_args()
+
+    if args.all:
+        PO_FILE_LIST = sorted(PO_DIR.glob('**/*.po'))
+    elif args.file:
+        if Path(args.file).is_file():
+            PO_FILE_LIST.append(Path(args.file))
+        else:
+            print("{}: file does not exist or is not a regular file".format(args.file), file=stderr)
+            return 1
 
     for po_file in PO_FILE_LIST:
         po = polib.pofile(po_file)
@@ -31,4 +46,4 @@ if __name__ == "__main__":
     try:
         main()
     except KeyboardInterrupt:
-        sys.exit(-1)
+        exit(-1)