Support PEP-415's Exception.__suppress_context__

PEP-415 states that `exception.__context__` should be suppressed
in traceback outputs, if `exception.__suppress_context__` is
`True`.

Now if a ``raise exception from None`` is caught by pytest,
pytest will no longer chain the context in the test report.

The algorithm in `FormattedExcinfo` now better matches the one
in `traceback.TracebackException`.

`Exception.__suppress_context__` is available in all of the
versions of Python 3 that are supported by pytest.

Fixes #2631.

AUTHORS

@@ -84,6 +84,7 @@ John Towler
 Jon Sonesen
 Jonas Obrist
 Jordan Guymon
+Jordan Moldow
 Joshua Bronson
 Jurko Gospodnetić
 Justyna Janczyszyn

_pytest/_code/code.py

@@ -679,7 +679,7 @@ class FormattedExcinfo(object):
                     e = e.__cause__
                     excinfo = ExceptionInfo((type(e), e, e.__traceback__)) if e.__traceback__ else None
                     descr = 'The above exception was the direct cause of the following exception:'
-                elif e.__context__ is not None:
+                elif (e.__context__ is not None and not e.__suppress_context__):
                     e = e.__context__
                     excinfo = ExceptionInfo((type(e), e, e.__traceback__)) if e.__traceback__ else None
                     descr = 'During handling of the above exception, another exception occurred:'

changelog/2631.feature

@@ -0,0 +1,4 @@
+Added support for `PEP-415's <https://www.python.org/dev/peps/pep-0415/>`_
+``Exception.__suppress_context__``. Now if a ``raise exception from None`` is
+caught by pytest, pytest will no longer chain the context in the test report.
+The behavior now matches Python's traceback behavior.

testing/code/test_excinfo.py

@@ -1095,6 +1095,36 @@ raise ValueError()
         assert line.endswith('mod.py')
         assert tw.lines[47] == ":15: AttributeError"
 
+    @pytest.mark.skipif("sys.version_info[0] < 3")
+    def test_exc_repr_with_raise_from_none_chain_suppression(self, importasmod):
+        mod = importasmod("""
+            def f():
+                try:
+                    g()
+                except Exception:
+                    raise AttributeError() from None
+            def g():
+                raise ValueError()
+        """)
+        excinfo = pytest.raises(AttributeError, mod.f)
+        r = excinfo.getrepr(style="long")
+        tw = TWMock()
+        r.toterminal(tw)
+        for line in tw.lines:
+            print(line)
+        assert tw.lines[0] == ""
+        assert tw.lines[1] == "    def f():"
+        assert tw.lines[2] == "        try:"
+        assert tw.lines[3] == "            g()"
+        assert tw.lines[4] == "        except Exception:"
+        assert tw.lines[5] == ">           raise AttributeError() from None"
+        assert tw.lines[6] == "E           AttributeError"
+        assert tw.lines[7] == ""
+        line = tw.get_write_msg(8)
+        assert line.endswith('mod.py')
+        assert tw.lines[9] == ":6: AttributeError"
+        assert len(tw.lines) == 10
+
     @pytest.mark.skipif("sys.version_info[0] < 3")
     @pytest.mark.parametrize('reason, description', [
         ('cause', 'The above exception was the direct cause of the following exception:'),