Fixed a crash when pretty-printing tags that were not created

   during initial parsing. [bug=1838903]

3 files changed, 31 insertions, 1 deletions

diff --git a/CHANGELOG b/CHANGELOG
index b19a486..a7d8260 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -6,6 +6,9 @@
   and position within a line (Tag.sourcepos).  Based on code by Chris
   Mayo. [bug=1742921]
 
+* Fixed a bug that made it impossible to pretty-print tags that were not
+   created during initial parsing. [bug=1838903]
+
 = 4.8.0 (20190720, "One Small Soup")
 
 This release focuses on making it easier to customize Beautiful Soup's
diff --git a/bs4/element.py b/bs4/element.py
index a57027e..07bd893 100644
--- a/bs4/element.py
+++ b/bs4/element.py
@@ -1173,7 +1173,10 @@ class Tag(PageElement):
         """Should this tag be pretty-printed?"""
         return (
             indent_level is not None
-            and self.name not in self.preserve_whitespace_tags
+            and (
+                not self.preserve_whitespace_tags
+                or self.name not in self.preserve_whitespace_tags
+            )
         )
 
     def prettify(self, encoding=None, formatter="minimal"):
diff --git a/bs4/tests/test_tree.py b/bs4/tests/test_tree.py
index e655dcc..c995c46 100644
--- a/bs4/tests/test_tree.py
+++ b/bs4/tests/test_tree.py
@@ -741,6 +741,30 @@ class TestPreviousSibling(SiblingTest):
         self.assertEqual(start.find_previous_sibling(text="nonesuch"), None)
 
 
+class TestTag(SoupTest):
+
+    # Test various methods of Tag.
+
+    def test__should_pretty_print(self):
+        # Test the rules about when a tag should be pretty-printed.
+        tag = self.soup("").new_tag("a_tag")
+
+        # No list of whitespace-preserving tags -> pretty-print
+        tag._preserve_whitespace_tags = None
+        self.assertEquals(True, tag._should_pretty_print(0))
+
+        # List exists but tag is not on the list -> pretty-print
+        tag.preserve_whitespace_tags = ["some_other_tag"]
+        self.assertEquals(True, tag._should_pretty_print(1))
+
+        # Indent level is None -> don't pretty-print
+        self.assertEquals(False, tag._should_pretty_print(None))
+        
+        # Tag is on the whitespace-preserving list -> don't pretty-print
+        tag.preserve_whitespace_tags = ["some_other_tag", "a_tag"]
+        self.assertEquals(False, tag._should_pretty_print(1))
+
+        
 class TestTagCreation(SoupTest):
     """Test the ability to create new tags."""
     def test_new_tag(self):