fix issue 655: crude workarounds around python2/3 exception leaks

CHANGELOG

@@ -1,6 +1,9 @@
 2.7.0.dev (compared to 2.6.4)
 -----------------------------
 
+- fix issue655: work around different ways that cause python2/3
+  to leak sys.exc_info into fixtures/tests causing failures in 3rd party code
+
 - fix issue615: assertion re-writing did not correctly escape % signs
   when formatting boolean operations, which tripped over mixing
   booleans with modulo operators.  Thanks to Tom Viner for the report,

_pytest/python.py

@@ -15,6 +15,8 @@ NOTSET = object()
 isfunction = inspect.isfunction
 isclass = inspect.isclass
 callable = py.builtin.callable
+# used to work around a python2 exception info leak
+exc_clear = getattr(sys, 'exc_clear', lambda: None)
 
 def getfslineno(obj):
     # xxx let decorators etc specify a sane ordering
@@ -1389,10 +1391,12 @@ class FixtureRequest(FuncargnamesCompatAttr):
                         cached_result = (self, [0], None)
                     return PseudoFixtureDef
                 raise
-            result = self._getfuncargvalue(fixturedef)
+        # remove indent to prevent the python3 exception
-            self._funcargs[argname] = result
+        # from leaking into the call
-            self._fixturedefs[argname] = fixturedef
+        result = self._getfuncargvalue(fixturedef)
-            return fixturedef
+        self._funcargs[argname] = result
+        self._fixturedefs[argname] = fixturedef
+        return fixturedef
 
     def _get_fixturestack(self):
         current = self
@@ -1439,6 +1443,9 @@ class FixtureRequest(FuncargnamesCompatAttr):
                     (scope, argname, self.scope, "\n".join(lines))))
             __tracebackhide__ = False
 
+        # clear sys.exc_info before invoking the fixture (python bug?)
+        # if its not explicitly cleared it will leak into the call
+        exc_clear()
         try:
             # call the fixture function
             val = fixturedef.execute(request=subrequest)

testing/python/fixture.py

@@ -261,6 +261,29 @@ class TestFillFixtures:
         ])
         assert "INTERNAL" not in result.stdout.str()
 
+    def test_fixture_excinfo_leak(self, testdir):
+        # on python2 sys.excinfo would leak into fixture executions
+        testdir.makepyfile("""
+            import sys
+            import traceback
+            import pytest
+
+            @pytest.fixture
+            def leak():
+                if sys.exc_info()[0]:  # python3 bug :)
+                    traceback.print_exc()
+                #fails
+                assert sys.exc_info() == (None, None, None)
+
+            def test_leak(leak):
+                if sys.exc_info()[0]:  # python3 bug :)
+                    traceback.print_exc()
+                assert sys.exc_info() == (None, None, None)
+        """)
+        result = testdir.runpytest()
+        assert result.ret == 0
+
+
 class TestRequestBasic:
     def test_request_attributes(self, testdir):
         item = testdir.getitem("""