fix(parser): raise ParserMatchError for malformed format strings (#1191)

arrow/parser.py

@@ -422,12 +422,21 @@ def parse(
         parts: _Parts = {}
         for token in fmt_tokens:
             value: Union[Tuple[str, str, str], str]
-            if token == "Do":
-                value = match.group("value")
-            elif token == "W":
-                value = (match.group("year"), match.group("week"), match.group("day"))
-            else:
-                value = match.group(token)
+            try:
+                if token == "Do":
+                    value = match.group("value")
+                elif token == "W":
+                    value = (
+                        match.group("year"),
+                        match.group("week"),
+                        match.group("day"),
+                    )
+                else:
+                    value = match.group(token)
+            except IndexError:
+                raise ParserMatchError(
+                    f"Failed to match {fmt!r} when parsing {datetime_string!r}."
+                )
 
             if value is None:
                 raise ParserMatchError(

tests/test_parser.py

@@ -529,6 +529,14 @@ def test_parse_with_leading_and_trailing_whitespace(self):
 
         assert self.parser.parse("      2016      ", "YYYY") == datetime(2016, 1, 1)
 
+    def test_parse_Do_indexerror_becomes_parser_match_error(self):
+        """Regression for #1191: On locales whose ordinal_day_re does not expose
+        a named 'value' capture group (e.g. GermanLocale), match.group('value')
+        previously raised a bare IndexError. It must raise ParserMatchError."""
+        de_parser = DateTimeParser(locale="de")
+        with pytest.raises(ParserMatchError):
+            de_parser.parse("1", "Do")
+
         assert self.parser.parse(
             "      2016-05-16 04:05:06.789120      ", "YYYY-MM-DD hh:mm:ss.S"
         ) == datetime(2016, 5, 16, 4, 5, 6, 789120)