support using py.test.raises in context manager style

--HG--
branch : trunk

py/_plugin/pytest_runner.py

@@ -358,13 +358,22 @@ def raises(ExpectedException, *args, **kwargs):
         if args[0] is a string: raise AssertionError if executing the
         the string in the calling scope does not raise expected exception. 
         for examples:
-        x = 5
-        raises(TypeError, lambda x: x + 'hello', x=x)
-        raises(TypeError, "x + 'hello'")
+        >>> x = 5
+        >>> raises(TypeError, lambda x: x + 'hello', x=x)
+        >>> raises(TypeError, "x + 'hello'")
+    
+        if no code/callable is given, it asumes you want a contextmanager 
+
+        >>> with raises(ZeroDivisionError):
+        ...    1/0
+        >>> with raises(TypeError):
+        ...     1 + 'a'
     """
     __tracebackhide__ = True
-    assert args
-    if isinstance(args[0], str):
+
+    if not args:
+        return RaisesContext(ExpectedException)
+    elif isinstance(args[0], str):
         code, = args
         assert isinstance(code, str)
         frame = sys._getframe(1)
@@ -391,6 +400,26 @@ def raises(ExpectedException, *args, **kwargs):
     raise ExceptionFailure(msg="DID NOT RAISE", 
                            expr=args, expected=ExpectedException) 
 
+
+class RaisesContext(object):
+
+    def __init__(self, ExpectedException):
+        self.ExpectedException = ExpectedException
+        self.excinfo = None
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *tp):
+        __tracebackhide__ = True
+        if tp[0] is None:
+            raise ExceptionFailure(msg="DID NOT RAISE",
+                                   expr=(),
+                                   expected=self.ExpectedException)
+        self.excinfo = py.code.ExceptionInfo(tp)
+        return issubclass(self.excinfo.type, self.ExpectedException)
+
+
 raises.Exception = ExceptionFailure
 
 def importorskip(modname, minversion=None):

testing/plugin/test_pytest_runner.py

@@ -340,6 +340,37 @@ class TestRaises:
         except py.test.raises.Exception:
             pass
 
+    @py.test.mark.skipif('sys.version < "2.5"')
+    def test_raises_as_contextmanager(self, testdir):
+        testdir.makepyfile("""
+            from __future__ import with_statement
+            import py
+
+            def test_simple():
+                with py.test.raises(ZeroDivisionError) as ctx:
+                    1/0
+
+                print ctx.excinfo
+                assert ctx.excinfo.type is ZeroDivisionError
+
+
+            def test_noraise():
+                with py.test.raises(py.test.raises.Exception):
+                    with py.test.raises(ValueError):
+                           int()
+
+            def test_raise_wrong_exception_passes_by():
+                with py.test.raises(ZeroDivisionError):
+                    with py.test.raises(ValueError):
+                           1/0
+        """)
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines([
+            '*3 passed*',
+        ])
+
+
+
 def test_pytest_exit():
     try:
         py.test.exit("hello")