Fix the stubborn test about cyclic references left by pytest.raises

In Python 2, a context manager's __exit__() leaves sys.exc_info with the exception values even when it was supposed
to suppress the exception, so we explicitly call sys.exc_clear() which removes the traceback and allow the object
to be released.

Also updated the test to not depend on the immediate destruction of the object but instead to ensure it is not being
tracked as a cyclic reference.

Fix #1965

Author: nicoddemus

CHANGELOG.rst

@@ -12,7 +12,10 @@
 * When loading plugins, import errors which contain non-ascii messages are now properly handled in Python 2 (`#1998`_).
   Thanks `@nicoddemus`_ for the PR.
 
-* Fixed memory leak in the RaisesContext (pytest.raises) (`#1965`_).
+* Fixed cyclic reference when ``pytest.raises`` is used in context-manager form (`#1965`_). Also as a
+  result of this fix, ``sys.exc_info()`` is left empty in both context-manager and function call usages.
+  Previously, ``sys.exc_info`` would contain the exception caught by the context manager,
+  even when the expected exception occurred.
   Thanks `@MSeifert04`_ for the report and the PR.
 
 * Fixed false-positives warnings from assertion rewrite hook for modules that were rewritten but

_pytest/python.py

@@ -1237,7 +1237,11 @@ def __exit__(self, *tp):
                 exc_type, value, traceback = tp
                 tp = exc_type, exc_type(value), traceback
         self.excinfo.__init__(tp)
-        return issubclass(self.excinfo.type, self.expected_exception)
+        suppress_exception = issubclass(self.excinfo.type, self.expected_exception)
+        if sys.version_info[0] == 2 and suppress_exception:
+            sys.exc_clear()
+        return suppress_exception
+
 
 # builtin pytest.approx helper
 

testing/python/raises.py

@@ -1,4 +1,6 @@
 import pytest
+import sys
+
 
 class TestRaises:
     def test_raises(self):
@@ -98,10 +100,13 @@ def test_custom_raise_message(self):
             assert False, "Expected pytest.raises.Exception"
 
     @pytest.mark.parametrize('method', ['function', 'with'])
-    def test_raises_memoryleak(self, method):
-        import weakref
+    def test_raises_cyclic_reference(self, method):
+        """
+        Ensure pytest.raises does not leave a reference cycle (#1965).
+        """
+        import gc
 
-        class T:
+        class T(object):
             def __call__(self):
                 raise ValueError
 
@@ -112,5 +117,12 @@ def __call__(self):
             with pytest.raises(ValueError):
                 t()
 
-        t = weakref.ref(t)
-        assert t() is None
+        # ensure both forms of pytest.raises don't leave exceptions in sys.exc_info()
+        assert sys.exc_info() == (None, None, None)
+
+        del t
+
+        # ensure the t instance is not stuck in a cyclic reference
+        for o in gc.get_objects():
+            assert type(o) is not T
+