Better exception messages for nested expressions; enhance assertRaise…

…sParseException to yield internal context manager (which has exception attribute)

pyparsing/helpers.py

@@ -537,6 +537,8 @@ def nested_expr(
     else:
         ret <<= Group(Suppress(opener) + ZeroOrMore(ret | content) + Suppress(closer))
     ret.set_name(f"nested {opener}{closer} expression")
+    # don't override error message from content expressions
+    ret.errmsg = None
     return ret
 
 

pyparsing/testing.py

@@ -241,12 +241,12 @@ def assertRaisesParseException(
             if expected_msg is not None:
                 if isinstance(expected_msg, str):
                     expected_msg = re.escape(expected_msg)
-                with self.assertRaisesRegex(exc_type, expected_msg, msg=msg):
-                    yield
+                with self.assertRaisesRegex(exc_type, expected_msg, msg=msg) as ctx:
+                    yield ctx
 
             else:
-                with self.assertRaises(exc_type, msg=msg):
-                    yield
+                with self.assertRaises(exc_type, msg=msg) as ctx:
+                    yield ctx
 
     @staticmethod
     def with_line_numbers(

tests/test_unit.py

@@ -10040,6 +10040,41 @@ def testForwardExceptionText(self):
             ff2 <<= wd
             ff2.parse_string("123")
 
+    def testForwardExceptionText2(self):
+        """
+        Test various expressions for error messages, under conditions in wrapped ParserElements
+        """
+        v = "(omit closing paren"
+        w = "('omit closing quote)"
+
+        for s, expr, expected in (
+            (v, pp.nested_expr(), "Expected ')'"),
+            (v, pp.Combine(pp.nested_expr(), adjacent=False), "Expected ')'"),
+            (v, pp.QuotedString("(", endQuoteChar=")"), "Expected quoted string, starting with ( ending with ), found '('"),
+            (w, pp.nested_expr(content=pp.sgl_quoted_string), "Expected ')'"),
+            ("", pp.nested_expr(), ""),
+            ("", pp.Word("A"), ""),
+        ):
+            print(repr(s))
+            print(expr)
+
+            with self.subTest("parse expr", expr=expr, s=s, expected=expected):
+                with self.assertRaisesParseException(expected_msg=expected) as ctx:
+                    expr.parse_string(s, parse_all=True)
+                print(ctx.exception)
+
+            with self.subTest("parse expr[1, ...]", expr=expr, s=s, expected=expected):
+                with self.assertRaisesParseException(expected_msg=expected) as ctx:
+                    expr[1, ...].parse_string(s, parse_all=True)
+                print(ctx.exception)
+
+            with self.subTest("parse DelimitedList(expr)", expr=expr, s=s, expected=expected):
+                with self.assertRaisesParseException(expected_msg=expected) as ctx:
+                    pp.DelimitedList(expr).parse_string(s, parse_all=True)
+                print(ctx.exception)
+
+            print()
+
     def testMiscellaneousExceptionBits(self):
         pp.ParserElement.verbose_stacktrace = True