Issue #4444: Allow assertRaises() to be used as a context handler.

Doc/library/unittest.rst

@@ -613,8 +613,8 @@ failures.
    equal, the test will fail with the explanation given by *msg*, or :const:`None`.
 
 
-.. method:: TestCase.assertRaises(exception, callable, ...)
-            TestCase.failUnlessRaises(exception, callable, ...)
+.. method:: TestCase.assertRaises(exception[, callable, ...])
+            TestCase.failUnlessRaises(exception[, callable, ...])
 
    Test that an exception is raised when *callable* is called with any positional
    or keyword arguments that are also passed to :meth:`assertRaises`.  The test
@@ -622,6 +622,13 @@ failures.
    fails if no exception is raised.  To catch any of a group of exceptions, a tuple
    containing the exception classes may be passed as *exception*.
 
+   .. versionchanged:: 2.2
+
+      If *callable* is omitted or None, returns a context manager so that the code
+      under test can be written inline rather than as a function::
+
+        with self.failUnlessRaises(some_error_class):
+            do_something()
 
 .. method:: TestCase.failIf(expr[, msg])
             TestCase.assertFalse(expr[, msg])

Lib/test/test_unittest.py

@@ -2284,6 +2284,43 @@ def test_AlmostEqual(self):
         self.assertRaises(AssertionError,
                           self.failIfAlmostEqual, 0, .1+.1j, places=0)
 
+    def test_assertRaises(self):
+        def _raise(e):
+            raise e
+        self.assertRaises(KeyError, _raise, KeyError)
+        self.assertRaises(KeyError, _raise, KeyError("key"))
+        try:
+            self.assertRaises(KeyError, lambda: None)
+        except AssertionError as e:
+            self.assert_("KeyError not raised" in e, str(e))
+        else:
+            self.fail("assertRaises() didn't fail")
+        try:
+            self.assertRaises(KeyError, _raise, ValueError)
+        except ValueError:
+            pass
+        else:
+            self.fail("assertRaises() didn't let exception pass through")
+        with self.assertRaises(KeyError):
+            raise KeyError
+        with self.assertRaises(KeyError):
+            raise KeyError("key")
+        try:
+            with self.assertRaises(KeyError):
+                pass
+        except AssertionError as e:
+            self.assert_("KeyError not raised" in e, str(e))
+        else:
+            self.fail("assertRaises() didn't fail")
+        try:
+            with self.assertRaises(KeyError):
+                raise ValueError
+        except ValueError:
+            pass
+        else:
+            self.fail("assertRaises() didn't let exception pass through")
+
+
 ######################################################################
 ## Main
 ######################################################################

Lib/unittest.py

@@ -174,6 +174,25 @@ def __repr__(self):
                (_strclass(self.__class__), self.testsRun, len(self.errors),
                 len(self.failures))
 
+class AssertRaisesContext:
+    def __init__(self, expected, test_case):
+        self.expected = expected
+        self.failureException = test_case.failureException
+    def __enter__(self):
+        pass
+    def __exit__(self, exc_type, exc_value, traceback):
+        if exc_type is None:
+            try:
+                exc_name = self.expected.__name__
+            except AttributeError:
+                exc_name = str(self.expected)
+            raise self.failureException(
+                "{0} not raised".format(exc_name))
+        if issubclass(exc_type, self.expected):
+            return True
+        # Let unexpected exceptions skip through
+        return False
+
 class TestCase:
     """A class whose instances are single test cases.
 
@@ -324,22 +343,25 @@ def failUnless(self, expr, msg=None):
         """Fail the test unless the expression is true."""
         if not expr: raise self.failureException, msg
 
-    def failUnlessRaises(self, excClass, callableObj, *args, **kwargs):
+    def failUnlessRaises(self, excClass, callableObj=None, *args, **kwargs):
         """Fail unless an exception of class excClass is thrown
            by callableObj when invoked with arguments args and keyword
            arguments kwargs. If a different type of exception is
            thrown, it will not be caught, and the test case will be
            deemed to have suffered an error, exactly as for an
            unexpected exception.
+
+           If called with callableObj omitted or None, will return a
+           context object used like this::
+
+                with self.failUnlessRaises(some_error_class):
+                    do_something()
         """
-        try:
+        context = AssertRaisesContext(excClass, self)
+        if callableObj is None:
+            return context
+        with context:
             callableObj(*args, **kwargs)
-        except excClass:
-            return
-        else:
-            if hasattr(excClass,'__name__'): excName = excClass.__name__
-            else: excName = str(excClass)
-            raise self.failureException, "%s not raised" % excName
 
     def failUnlessEqual(self, first, second, msg=None):
         """Fail if the two objects are unequal as determined by the '=='

Misc/NEWS

@@ -91,6 +91,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #4444: Allow assertRaises() to be used as a context handler, so that
+  the code under test can be written inline if more practical.
+
 - Issue #4739: Add pydoc help topics for symbols, so that e.g. help('@')
   works as expected in the interactive environment.