Storing sys.last_traceback: test, docs and changelog

CHANGELOG

@@ -1,6 +1,10 @@
 2.7.0.dev (compared to 2.6.4)
 -----------------------------
 
+- On failure, the ``sys.last_value``, ``sys.last_type`` and 
+  ``sys.last_traceback`` are set, so that a user can inspect the error
+  via postmortem debugging.
+
 - fix issue616: conftest.py files and their contained fixutres are now
   properly considered for visibility, independently from the exact
   current working directory and test arguments that are used.

doc/en/usage.txt

@@ -87,6 +87,9 @@ failure situation::
     py.test -x --pdb   # drop to PDB on first failure, then end test session
     py.test --pdb --maxfail=3  # drop to PDB for first three failures
 
+Note that on any failure the exception information is stored on 
+``sys.last_traceback``. In interactive use, this allows one to drop
+into postmortem debugging with any debug tool.
 
 Setting a breakpoint / aka ``set_trace()``
 ----------------------------------------------------

testing/test_runner.py

@@ -525,3 +525,18 @@ def test_makereport_getsource(testdir):
     result = testdir.runpytest()
     assert 'INTERNALERROR' not in result.stdout.str()
     result.stdout.fnmatch_lines(['*else: assert False*'])
+
+
+def test_store_except_info_on_eror(testdir):
+    # Simulate item that raises a specific exception
+    class ItemThatRaises:
+        def runtest(self):
+            raise IndexError('TEST')
+    try:
+        runner.pytest_runtest_call(ItemThatRaises())
+    except IndexError:
+        pass
+    # Check that exception info is stored on sys
+    assert sys.last_type is IndexError
+    assert sys.last_value.args[0] == 'TEST'
+    assert sys.last_traceback