Accept pointer as argument in commandline utility

bin/jsonpointer

@@ -12,8 +12,17 @@ import argparse
 
 parser = argparse.ArgumentParser(
     description='Resolve a JSON pointer on JSON files')
-parser.add_argument('POINTER', type=argparse.FileType('r'),
-                    help='File containing a JSON pointer expression')
+
+# Accept pointer as argument or as file
+ptr_group = parser.add_mutually_exclusive_group(required=True)
+
+ptr_group.add_argument('-f', '--pointer-file', type=argparse.FileType('r'),
+                       nargs='?',
+                       help='File containing a JSON pointer expression')
+
+ptr_group.add_argument('POINTER',  type=str, nargs='?',
+                       help='A JSON pointer expression')
+
 parser.add_argument('FILE', type=argparse.FileType('r'), nargs='+',
                     help='Files for which the pointer should be resolved')
 parser.add_argument('--indent', type=int, default=None,
@@ -29,10 +38,24 @@ def main():
         sys.exit(1)
 
 
+def parse_pointer(args):
+    if args.POINTER:
+        ptr = args.POINTER
+    elif args.pointer_file:
+        ptr = args.pointer_file.read().strip()
+    else:
+        parser.print_usage()
+        sys.exit(1)
+
+    return ptr
+
+
 def resolve_files():
     """ Resolve a JSON pointer on JSON files """
     args = parser.parse_args()
-    ptr = json.load(args.POINTER)
+
+    ptr = parse_pointer(args)
+
     for f in args.FILE:
         doc = json.load(f)
         try: