Fix #470 - Removed "" from the list of values that ParseResults will not save with a results name

3 files changed, 77 insertions, 8 deletions

diff --git a/CHANGES b/CHANGES
index 2886034..a73e2c0 100644
--- a/CHANGES
+++ b/CHANGES
@@ -4,15 +4,25 @@ Change Log
 
 Version 3.1.0a2 - (in development)
 ----------------------------------
-Fixed bug in `SkipTo` where ignore expressions were not properly handled while
-scanning for the target expression. Issue #475, reported by elkniwt, thanks
-(this bug has been there for a looooong time!).
+- Fixed bug when parse actions returned an empty string for an expression that
+  had a results name, that the results name was not saved. That is:
 
-Updated ci.yml permissions to limit default access to source - submitted by Joyce
-Brum of Google. Thanks so much!
+      expr = Literal("X").add_parse_action(lambda tokens: "")("value")
+      result = expr.parse_string("X")
+      print(result["value"])
 
-Updated the lucene_grammar.py example (better support for '*' and '?' wildcards)
-and corrected the test cases - brought to my attention by Elijah Nicol, good catch!
+  would raise a `KeyError`. Now empty strings will be saved with the associated
+  results name. Raised in Issue #470 by Nicco Kunzmann, thank you.
+
+- Fixed bug in `SkipTo` where ignore expressions were not properly handled while
+  scanning for the target expression. Issue #475, reported by elkniwt, thanks
+  (this bug has been there for a looooong time!).
+
+- Updated ci.yml permissions to limit default access to source - submitted by Joyce
+  Brum of Google. Thanks so much!
+
+- Updated the lucene_grammar.py example (better support for '*' and '?' wildcards)
+  and corrected the test cases - brought to my attention by Elijah Nicol, good catch!
 
 
 Version 3.1.0a1 - March, 2023
diff --git a/pyparsing/results.py b/pyparsing/results.py
index 1134456..92f89f4 100644
--- a/pyparsing/results.py
+++ b/pyparsing/results.py
@@ -77,7 +77,7 @@ class ParseResults:
         - year: '1999'
     """
 
-    _null_values: Tuple[Any, ...] = (None, [], "", ())
+    _null_values: Tuple[Any, ...] = (None, [], ())
 
     _name: str
     _parent: "ParseResults"
diff --git a/tests/test_unit.py b/tests/test_unit.py
index 0f1cb5a..8c98a3e 100644
--- a/tests/test_unit.py
+++ b/tests/test_unit.py
@@ -2754,6 +2754,65 @@ class Test02_WithoutPackrat(ppt.TestParseResultsAsserts, TestCase):
         with self.assertRaises(AttributeError):
             result.__xyz__
 
+    def testParseResultsNamedResultWithEmptyString(self):
+        # from Issue #470
+
+        # Check which values can be returned from a parse action
+        for test_value, expected_in_result_by_name in [
+            ("x", True),
+            ("", True),
+            (True, True),
+            (False, True),
+            (1, True),
+            (0, True),
+            (None, True),
+            (b"", True),
+            (b"a", True),
+            ([], False),
+            ((), False),
+        ]:
+            msg = (f"value = {test_value!r},"
+                   f" expected X {'not ' if not expected_in_result_by_name else ''}in result")
+            with self.subTest(msg):
+                print(msg)
+                grammar = ((pp.Suppress("a") + pp.ZeroOrMore("x"))
+                           .add_parse_action(lambda p: test_value)
+                           .set_results_name("X"))
+                result = grammar.parse_string("a")
+                print(result.dump())
+                if expected_in_result_by_name:
+                    self.assertIn("X", result, f"Expected X not found for parse action value {test_value!r}")
+                    print(repr(result["X"]))
+                else:
+                    self.assertNotIn("X", result, f"Unexpected X found for parse action value {test_value!r}")
+                    with self.assertRaises(KeyError):
+                        print(repr(result["X"]))
+                print()
+
+        # Do not add a parse result.
+        msg = "value = <no parse action defined>, expected X in result"
+        with self.subTest(msg):
+            print(msg)
+            grammar = (pp.Suppress("a") + pp.ZeroOrMore("x")).set_results_name("X")
+            result = grammar.parse_string("a")
+            print(result.dump())
+            self.assertIn("X", result, f"Expected X not found with no parse action")
+            print()
+
+        # Test by directly creating a ParseResults
+        print("Create empty string value directly")
+        result = pp.ParseResults("", name="X")
+        print(result.dump())
+        self.assertIn("X", result, "failed to construct ParseResults with named value using empty string")
+        print(repr(result["X"]))
+        print()
+
+        print("Create empty string value from a dict")
+        result = pp.ParseResults.from_dict({"X": ""})
+        print(result.dump())
+        self.assertIn("X", result, "failed to construct ParseResults with named value using from_dict")
+        print(repr(result["X"]))
+
     def testMatchOnlyAtCol(self):
         """successfully use matchOnlyAtCol helper function"""