gh-69113: Fix doctest to report line numbers for __test__ strings

Enhanced the _find_lineno method in doctest to correctly identify and
report line numbers for doctests defined in __test__ dictionaries when
formatted as triple-quoted strings.

Uses a hybrid approach:
1. Fast path: find a line that's unique in the source file
2. Fallback: multi-line matching if no unique line exists

Previously, doctest would report "line None" for __test__ dictionary
strings, making it difficult to debug failing tests.

Co-Authored-By: Jurjen N.E. Bos <jneb@users.sourceforge.net>
Co-Authored-By: R. David Murray <rdmurray@bitdance.com>

Author: 3 people

Lib/doctest.py

@@ -1167,6 +1167,34 @@ def _find_lineno(self, obj, source_lines):
                 if pat.match(source_lines[lineno]):
                     return lineno
 
+        # Handle __test__ string doctests formatted as triple-quoted strings.
+        # Try two approaches:
+        # 1. Fast path: find a line that's unique in the source file
+        # 2. Fallback: multi-line matching if no unique line exists
+        if isinstance(obj, str) and source_lines is not None:
+            obj_lines = obj.splitlines(keepends=True)
+
+            # Fast path: find a line unique in the source file
+            for offset, line in enumerate(obj_lines):
+                if source_lines.count(line) == 1:
+                    return source_lines.index(line) - offset
+
+            # Fallback: multi-line matching for when no single line is unique
+            # Skip the first line (may be on same line as opening quotes)
+            # and any blank lines to find a meaningful line to match
+            index = 1
+            while index < len(obj_lines) and not obj_lines[index].strip():
+                index += 1
+            if index < len(obj_lines):
+                for lineno, source_line in enumerate(source_lines):
+                    if obj_lines[index] == source_line:
+                        # Verify subsequent lines also match
+                        for i in range(index + 1, len(obj_lines) - 1):
+                            if obj_lines[i] != source_lines[lineno + i - index]:
+                                break
+                        else:
+                            return lineno - index
+
         # We couldn't find the line number.
         return None
 

Lib/test/test_doctest/test_doctest.py

@@ -833,6 +833,121 @@ def test_empty_namespace_package(self):
             self.assertEqual(len(include_empty_finder.find(mod)), 1)
             self.assertEqual(len(exclude_empty_finder.find(mod)), 0)
 
+    def test_lineno_of_test_dict_strings(self):
+        """Test that line numbers are correctly found for __test__ dict strings."""
+        # Create a temporary module with a __test__ dict containing triple-quoted strings
+        module_content = '''\
+"""Module docstring."""
+
+def dummy_function():
+    """Dummy function docstring."""
+    pass
+
+__test__ = {
+    'test_string': """
+    This is a test string.
+    >>> 1 + 1
+    2
+    """,
+}
+'''
+        with tempfile.TemporaryDirectory() as tmpdir:
+            module_path = os.path.join(tmpdir, 'test_module_lineno.py')
+            with open(module_path, 'w') as f:
+                f.write(module_content)
+
+            sys.path.insert(0, tmpdir)
+            try:
+                # Import the module
+                import test_module_lineno
+
+                # Use DocTestFinder to find tests
+                finder = doctest.DocTestFinder()
+                tests = finder.find(test_module_lineno)
+
+                # Find the test from __test__ dict
+                test_dict_test = None
+                for test in tests:
+                    if '__test__' in test.name:
+                        test_dict_test = test
+                        break
+
+                # Assert that we found the test and it has a valid line number
+                self.assertIsNotNone(test_dict_test, "__test__ dict test not found")
+                # The line number should not be None - this is what the bug fix addresses
+                self.assertIsNotNone(test_dict_test.lineno,
+                                     "Line number should not be None for __test__ dict strings")
+                # The line number should be around line 9 (where the triple-quoted string starts)
+                # Allowing some tolerance for exact line number
+                self.assertGreater(test_dict_test.lineno, 0,
+                                   "Line number should be positive")
+            finally:
+                # Clean up
+                if 'test_module_lineno' in sys.modules:
+                    del sys.modules['test_module_lineno']
+                sys.path.pop(0)
+
+    def test_lineno_fallback_multiline_matching(self):
+        """Test fallback to multi-line matching when no unique line exists."""
+        # Create a module where the same lines appear multiple times,
+        # forcing the fallback to multi-line matching
+        module_content = '''\
+"""Module docstring."""
+
+# These lines appear in both __test__ entries
+# >>> x = 1
+# >>> x
+# 1
+
+__test__ = {
+    'test_one': """
+    >>> x = 1
+    >>> x
+    1
+    """,
+    'test_two': """
+    >>> x = 1
+    >>> x
+    2
+    """,
+}
+'''
+        with tempfile.TemporaryDirectory() as tmpdir:
+            module_path = os.path.join(tmpdir, 'test_module_fallback.py')
+            with open(module_path, 'w') as f:
+                f.write(module_content)
+
+            sys.path.insert(0, tmpdir)
+            try:
+                import test_module_fallback
+
+                finder = doctest.DocTestFinder()
+                tests = finder.find(test_module_fallback)
+
+                # Find both __test__ entries
+                test_one = None
+                test_two = None
+                for test in tests:
+                    if 'test_one' in test.name:
+                        test_one = test
+                    elif 'test_two' in test.name:
+                        test_two = test
+
+                # Both should have valid line numbers found via fallback
+                self.assertIsNotNone(test_one, "test_one not found")
+                self.assertIsNotNone(test_two, "test_two not found")
+                self.assertIsNotNone(test_one.lineno,
+                                     "Line number should not be None for test_one")
+                self.assertIsNotNone(test_two.lineno,
+                                     "Line number should not be None for test_two")
+                # They should have different line numbers
+                self.assertNotEqual(test_one.lineno, test_two.lineno,
+                                    "test_one and test_two should have different line numbers")
+            finally:
+                if 'test_module_fallback' in sys.modules:
+                    del sys.modules['test_module_fallback']
+                sys.path.pop(0)
+
 def test_DocTestParser(): r"""
 Unit tests for the `DocTestParser` class.
 
@@ -2434,7 +2549,8 @@ def test_DocTestSuite_errors():
          <BLANKLINE>
          >>> print(result.failures[1][1]) # doctest: +ELLIPSIS
          Traceback (most recent call last):
-           File "...sample_doctest_errors.py", line None, in test.test_doctest.sample_doctest_errors.__test__.bad
+           File "...sample_doctest_errors.py", line 37, in test.test_doctest.sample_doctest_errors.__test__.bad
+             >...>> 2 + 2
          AssertionError: Failed example:
              2 + 2
          Expected:
@@ -2464,7 +2580,8 @@ def test_DocTestSuite_errors():
          <BLANKLINE>
          >>> print(result.errors[1][1]) # doctest: +ELLIPSIS
          Traceback (most recent call last):
-           File "...sample_doctest_errors.py", line None, in test.test_doctest.sample_doctest_errors.__test__.bad
+           File "...sample_doctest_errors.py", line 39, in test.test_doctest.sample_doctest_errors.__test__.bad
+             >...>> 1/0
            File "<doctest test.test_doctest.sample_doctest_errors.__test__.bad[1]>", line 1, in <module>
              1/0
              ~^~
@@ -3256,15 +3373,15 @@ def test_testmod_errors(): r"""
             ~^~
         ZeroDivisionError: division by zero
     **********************************************************************
-    File "...sample_doctest_errors.py", line ?, in test.test_doctest.sample_doctest_errors.__test__.bad
+    File "...sample_doctest_errors.py", line 37, in test.test_doctest.sample_doctest_errors.__test__.bad
     Failed example:
         2 + 2
     Expected:
         5
     Got:
         4
     **********************************************************************
-    File "...sample_doctest_errors.py", line ?, in test.test_doctest.sample_doctest_errors.__test__.bad
+    File "...sample_doctest_errors.py", line 39, in test.test_doctest.sample_doctest_errors.__test__.bad
     Failed example:
         1/0
     Exception raised:

Misc/NEWS.d/next/Library/2025-11-16-04-40-06.gh-issue-69113.Xy7Fmn.rst

@@ -0,0 +1 @@
+Fix :mod:`doctest` to correctly report line numbers for doctests in ``__test__`` dictionary when formatted as triple-quoted strings by finding unique lines in the string and matching them in the source file.