gh-70280: allow using custom sys.displayhook's with doctest

Doc/library/doctest.rst

@@ -1631,6 +1631,10 @@ DocTestRunner objects
       output checker, and the results are formatted by the
       :meth:`!DocTestRunner.report_\*` methods.
 
+      .. versionchanged:: 3.13
+          Added support for testing examples with a customized
+          :func:`sys.displayhook` value.  Previously, this wasn't allowed and
+          the method was always using the :func:`sys.__displayhook__`.
 
    .. method:: summarize(verbose=None)
 

Lib/doctest.py

@@ -1565,9 +1565,8 @@ def out(s):
         self.save_linecache_getlines = linecache.getlines
         linecache.getlines = self.__patched_linecache_getlines
 
-        # Make sure sys.displayhook just prints the value to stdout
+        # Make sure sys.displayhook restored at the end
         save_displayhook = sys.displayhook
-        sys.displayhook = sys.__displayhook__
         saved_can_colorize = _colorize.can_colorize
         _colorize.can_colorize = lambda *args, **kwargs: False
         color_variables = {"PYTHON_COLORS": None, "FORCE_COLOR": None}

Lib/test/test_doctest/test_doctest.py

@@ -1268,33 +1268,46 @@ def exceptions(): r"""
     >>> _colorize.COLORIZE = save_colorize
 """
     def displayhook(): r"""
-Test that changing sys.displayhook doesn't matter for doctest.
+Test changing sys.displayhook.
+
+Run with a custom displayhook:
 
     >>> import sys
-    >>> orig_displayhook = sys.displayhook
     >>> def my_displayhook(x):
     ...     print('hi!')
+    ...     sys.__displayhook__(x)
     >>> sys.displayhook = my_displayhook
+    >>> 3
+    hi!
+    3
     >>> def f():
     ...     '''
     ...     >>> 3
+    ...     hi!
     ...     3
     ...     '''
     >>> test = doctest.DocTestFinder().find(f)[0]
-    >>> r = doctest.DocTestRunner(verbose=False).run(test)
-    >>> post_displayhook = sys.displayhook
-
-    We need to restore sys.displayhook now, so that we'll be able to test
-    results.
-
-    >>> sys.displayhook = orig_displayhook
+    >>> doctest.DocTestRunner(verbose=False).run(test)
+    hi!
+    TestResults(failed=0, attempted=1)
+    >>> sys.displayhook = sys.__displayhook__
+    >>> 3
+    3
 
-    Ok, now we can check that everything is ok.
+Test changing displayhook in the doctest:
 
-    >>> r
-    TestResults(failed=0, attempted=1)
-    >>> post_displayhook is my_displayhook
-    True
+    >>> def g():
+    ...     '''
+    ...     >>> import sys
+    ...     >>> sys.displayhook = lambda x: print('spam')
+    ...     >>> 3
+    ...     spam
+    ...     '''
+    >>> test = doctest.DocTestFinder().find(g)[0]
+    >>> doctest.DocTestRunner(verbose=False).run(test)
+    TestResults(failed=0, attempted=3)
+    >>> 42
+    42
 """
     def optionflags(): r"""
 Tests of `DocTestRunner`'s option flag handling.

Misc/NEWS.d/next/Library/2021-04-27-08-36-50.bpo-26092.1qLzp5.rst

@@ -0,0 +1,2 @@
+Allow using custom sys.displayhook's with doctest.  Patch by Sergey B
+Kirpichev.