bpo-41003: Fix test_copyreg when numpy is installed (GH-20935)

Fix test_copyreg when numpy is installed: test.pickletester now
saves/restores warnings.filters when importing numpy, to ignore
filters installed by numpy.

Add the save_restore_warnings_filters() function to the
test.support.warnings_helper module.

Lib/distutils/tests/__init__.py

@@ -15,26 +15,25 @@ by import rather than matching pre-defined names.
 import os
 import sys
 import unittest
-import warnings
 from test.support import run_unittest
+from test.support.warnings_helper import save_restore_warnings_filters
 
 
 here = os.path.dirname(__file__) or os.curdir
 
 
 def test_suite():
-    old_filters = warnings.filters[:]
     suite = unittest.TestSuite()
     for fn in os.listdir(here):
         if fn.startswith("test") and fn.endswith(".py"):
             modname = "distutils.tests." + fn[:-3]
-            __import__(modname)
+            # bpo-40055: Save/restore warnings filters to leave them unchanged.
+            # Importing tests imports docutils which imports pkg_resources
+            # which adds a warnings filter.
+            with save_restore_warnings_filters():
+                __import__(modname)
             module = sys.modules[modname]
             suite.addTest(module.test_suite())
-    # bpo-40055: Save/restore warnings filters to leave them unchanged.
-    # Importing tests imports docutils which imports pkg_resources which adds a
-    # warnings filter.
-    warnings.filters[:] = old_filters
     return suite
 
 

Lib/test/pickletester.py

@@ -21,20 +21,26 @@ try:
 except ImportError:
     _testbuffer = None
 
-try:
-    import numpy as np
-except ImportError:
-    np = None
-
 from test import support
 from test.support import (
     TestFailed, TESTFN, run_with_locale, no_tracing,
     _2G, _4G, bigmemtest, forget,
     )
 from test.support import threading_helper
+from test.support.warnings_helper import save_restore_warnings_filters
 
 from pickle import bytes_types
 
+
+# bpo-41003: Save/restore warnings filters to leave them unchanged.
+# Ignore filters installed by numpy.
+try:
+    with save_restore_warnings_filters():
+        import numpy as np
+except ImportError:
+    np = None
+
+
 requires_32b = unittest.skipUnless(sys.maxsize < 2**32,
                                    "test is only meaningful on 32-bit builds")
 

Lib/test/support/warnings_helper.py

@@ -178,3 +178,12 @@ def _filterwarnings(filters, quiet=False):
     if missing:
         raise AssertionError("filter (%r, %s) did not catch any warning" %
                              missing[0])
+
+
+@contextlib.contextmanager
+def save_restore_warnings_filters():
+    old_filters = warnings.filters[:]
+    try:
+        yield
+    finally:
+        warnings.filters[:] = old_filters

Misc/NEWS.d/next/Tests/2020-06-17-15-07-14.bpo-41003.tiH_Fy.rst

@@ -0,0 +1,3 @@
+Fix ``test_copyreg`` when ``numpy`` is installed: ``test.pickletester`` now
+saves/restores warnings filters when importing ``numpy``, to ignore filters
+installed by ``numpy``.