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. The implementation finds unique lines
in the test string and matches them in the source file to calculate the
correct starting line number.

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>

Author: CuriousLearner

Lib/doctest.py

@@ -1167,6 +1167,15 @@ def _find_lineno(self, obj, source_lines):
                 if pat.match(source_lines[lineno]):
                     return lineno
 
+        # Handle __test__ string doctests by finding a unique line
+        # in the string and matching it in the source file
+        if isinstance(obj, str) and source_lines is not None:
+            # This will find __test__ string doctests if and only if the string
+            # contains any unique line.
+            for offset, line in enumerate(obj.splitlines(keepends=True)):
+                if source_lines.count(line) == 1:
+                    return source_lines.index(line) - offset
+
         # We couldn't find the line number.
         return None
 

Lib/test/test_doctest/test_doctest.py

@@ -833,6 +833,60 @@ 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_DocTestParser(): r"""
 Unit tests for the `DocTestParser` class.
 
@@ -2434,7 +2488,8 @@ 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 +2519,8 @@ 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 +3312,15 @@ 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.