Use inspect.isroutine() in DocTest's lineno computation

Previously, DocTest's lineno of `functools.cache()`-decorated functions
was not properly returned (None was returned) because the underlying
computation, in `DocTest._find_lineno()`, relied on
`inspect.isfunction()` which does not consider the decorated result as a
function.

We now use the more generic `inspect.isroutine()`, as elsewhere
in doctest's logic, thus fixing lineno computation for
functools.cache()-decorated functions.

Author: dlax

Lib/doctest.py

@@ -1141,7 +1141,7 @@ def _find_lineno(self, obj, source_lines):
         if inspect.ismethod(obj): obj = obj.__func__
         if isinstance(obj, property):
             obj = obj.fget
-        if inspect.isfunction(obj) and getattr(obj, '__doc__', None):
+        if inspect.isroutine(obj) and getattr(obj, '__doc__', None):
             # We don't use `docstring` var here, because `obj` can be changed.
             obj = inspect.unwrap(obj)
             try:

Lib/test/test_doctest/doctest_lineno.py

@@ -76,3 +76,21 @@ def property_with_doctest(self):
 @decorator
 def func_with_docstring_wrapped():
     """Some unrelated info."""
+
+
+# https://github.com/python/cpython/issues/136914
+import functools
+
+
+@functools.cache
+def cached_func_with_doctest(value):
+    """
+    >>> cached_func_with_doctest(1)
+    -1
+    """
+    return -value
+
+
+@functools.cache
+def cached_func_without_docstring(value):
+    return value + 1

Lib/test/test_doctest/test_doctest.py

@@ -687,6 +687,8 @@ def basics(): r"""
        45  test.test_doctest.doctest_lineno.MethodWrapper.method_with_doctest
      None  test.test_doctest.doctest_lineno.MethodWrapper.method_without_docstring
        61  test.test_doctest.doctest_lineno.MethodWrapper.property_with_doctest
+       85  test.test_doctest.doctest_lineno.cached_func_with_doctest
+     None  test.test_doctest.doctest_lineno.cached_func_without_docstring
         4  test.test_doctest.doctest_lineno.func_with_docstring
        77  test.test_doctest.doctest_lineno.func_with_docstring_wrapped
        12  test.test_doctest.doctest_lineno.func_with_doctest

Misc/NEWS.d/next/Library/2025-07-21-15-40-00.gh-issue-136914.-GNG-d.rst

@@ -0,0 +1,2 @@
+Fix retrieval of :attr:`DocTest.lineno` for :func:`functools.cache`-decorated
+functions.