From b0839c91da7b29a7412c373326e1ab05f23a5afc Mon Sep 17 00:00:00 2001
From: Sergey B Kirpichev <skirpichev@gmail.com>
Date: Mon, 26 Apr 2021 10:27:16 +0300
Subject: [PATCH] gh-70280: allow using custom sys.displayhook's with doctest

This partially reverts patch from https://bugs.python.org/issue8048

Now it's possible to test interactive examples in applications, which
use cusomized sys.displayhook, e.g. SymPy, without nasty workarrounds
(i.e. overriding the sys.__displayhook__ value before each tests).
---
 Doc/library/doctest.rst                       |  4 ++
 Lib/doctest.py                                |  3 +-
 Lib/test/test_doctest.py                      | 41 ++++++++++++-------
 .../2021-04-27-08-36-50.bpo-26092.1qLzp5.rst  |  2 +
 4 files changed, 34 insertions(+), 16 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2021-04-27-08-36-50.bpo-26092.1qLzp5.rst

diff --git a/Doc/library/doctest.rst b/Doc/library/doctest.rst
index ad013944ce3ca3..349410b8111836 100644
--- a/Doc/library/doctest.rst
+++ b/Doc/library/doctest.rst
@@ -1562,6 +1562,10 @@ 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)
 
diff --git a/Lib/doctest.py b/Lib/doctest.py
index 2f14aa08334895..fa33e9d44b2f70 100644
--- a/Lib/doctest.py
+++ b/Lib/doctest.py
@@ -1533,9 +1533,8 @@ 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__
 
         try:
             return self.__run(test, compileflags, out)
diff --git a/Lib/test/test_doctest.py b/Lib/test/test_doctest.py
index cb4e2157bb228b..4d07a39cac3915 100644
--- a/Lib/test/test_doctest.py
+++ b/Lib/test/test_doctest.py
@@ -1220,33 +1220,46 @@ def exceptions(): r"""
     TestResults(failed=1, attempted=1)
 """
     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.
diff --git a/Misc/NEWS.d/next/Library/2021-04-27-08-36-50.bpo-26092.1qLzp5.rst b/Misc/NEWS.d/next/Library/2021-04-27-08-36-50.bpo-26092.1qLzp5.rst
new file mode 100644
index 00000000000000..e4c12bdfcf1093
--- /dev/null
+++ b/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.