Avoid nested bags by default

bagit.py

@@ -138,13 +138,27 @@ def find_locale_dir():
 UNICODE_BYTE_ORDER_MARK = "\ufeff"
 
 
+def is_bag(bag_dir):
+    """
+    Return a boolean whether the given directory is already a bag.
+    """
+    try:
+        Bag(bag_dir)
+        return True
+    except BagError:
+        return False
+
+
 def make_bag(
-    bag_dir, bag_info=None, processes=1, checksums=None, checksum=None, encoding="utf-8"
+    bag_dir, bag_info=None, processes=1, checksums=None, checksum=None, encoding="utf-8", allow_nested_bag=False
 ):
     """
     Convert a given directory into a bag. You can pass in arbitrary
     key/value pairs to put into the bag-info.txt metadata file as
     the bag_info dictionary.
+
+    By default creating a bag of directory that is already a bag will raise an error.
+    Set allow_nested_bag to allow creation of nested bags.
     """
 
     if checksum is not None:
@@ -167,6 +181,13 @@ def make_bag(
             _("Bagging a parent of the current directory is not supported")
         )
 
+    if not allow_nested_bag:
+        if is_bag(bag_dir):
+            raise RuntimeError(
+                _(f"The directory '{bag_dir}' is already a bag. "
+                  "Use allow_nested_bag=True to allow creation of a nested bag.")
+            )
+
     LOGGER.info(_("Creating bag for directory %s"), bag_dir)
 
     if not os.path.isdir(bag_dir):

test.py

@@ -97,6 +97,31 @@ def test_make_bag_md5_sha1_sha256_manifest(self):
         # check valid with three manifests
         self.assertTrue(self.validate(bag, fast=True))
 
+    def test_is_bag_false_initially(self):
+        self.assertFalse(bagit.is_bag(self.tmpdir))
+
+    def test_is_bag_true_after_make_bag(self):
+        bagit.make_bag(self.tmpdir)
+        self.assertTrue(bagit.is_bag(self.tmpdir))
+
+    def test_make_nested_bag_without_flag(self):
+        bagit.make_bag(self.tmpdir)
+
+        with self.assertRaises(RuntimeError) as ctx:
+            tmpdir = self.tmpdir
+            bagit.make_bag(tmpdir, allow_nested_bag=False)
+
+        expected_msg = (f"The directory '{tmpdir}' is already a bag. "
+                        "Use allow_nested_bag=True to allow creation of a nested bag.")
+        self.assertEqual(str(ctx.exception), expected_msg)
+
+    def test_make_nested_bag_with_flag(self):
+        bagit.make_bag(self.tmpdir)
+
+        bag = bagit.make_bag(self.tmpdir, allow_nested_bag=True)
+
+        self.assertIsInstance(bag, bagit.Bag)
+
     def test_validate_flipped_bit(self):
         bag = bagit.make_bag(self.tmpdir)
         readme = j(self.tmpdir, "data", "README")