Add bad-exception-context warning for checking that `raise ... from ...` sets a proper exception context.

4 files changed, 49 insertions, 2 deletions

diff --git a/ChangeLog b/ChangeLog
index f5b7192..6ed61d6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,11 @@
 ChangeLog for Pylint
 ====================
 
+--
+    * Add new warning 'bad-exception-context', checking
+      that `raise ... from ...` uses a proper exception context
+      (None or an exception).
+
 2013-12-22  --  1.1.0
     * Add new check for use of deprecated pragma directives "pylint:disable-msg"
       or "pylint:enable-msg" (I0022, deprecated-pragma) which was previously
diff --git a/checkers/exceptions.py b/checkers/exceptions.py
index 9843001..693e0f5 100644
--- a/checkers/exceptions.py
+++ b/checkers/exceptions.py
@@ -25,7 +25,7 @@ from pylint.checkers import BaseChecker
 from pylint.checkers.utils import is_empty, is_raising, check_messages
 from pylint.interfaces import IAstroidChecker
 
-
+PY3K = sys.version_info >= (3, 0)
 OVERGENERAL_EXCEPTIONS = ('Exception',)
 
 MSGS = {
@@ -38,6 +38,13 @@ MSGS = {
               'raising-bad-type',
               'Used when something which is neither a class, an instance or a \
               string is raised (i.e. a `TypeError` will be raised).'),
+    'E0703': ('Exception context set to something which is not an '
+              'exception, nor None',
+              'bad-exception-context',
+              'Used when using the syntax "raise ... from ...", '
+              'where the exception context is not an exception, '
+              'nor None.',
+              {'minversion': (3, 0)}),
     'E0710': ('Raising a new style class which doesn\'t inherit from BaseException',
               'raising-non-exception',
               'Used when a new style class which doesn\'t inherit from \
@@ -109,12 +116,28 @@ class ExceptionsChecker(BaseChecker):
                 ),
                )
 
-    @check_messages('W0701', 'W0710', 'E0702', 'E0710', 'E0711')
+    @check_messages('W0701', 'W0710', 'E0702', 'E0710', 'E0711',
+                    'bad-exception-context')
     def visit_raise(self, node):
         """visit raise possibly inferring value"""
         # ignore empty raise
         if node.exc is None:
             return
+        if PY3K and node.cause:
+            if isinstance(node.cause, astroid.Const):
+                if node.cause.value is not None:
+                    self.add_message('bad-exception-context',
+                                     node=node)
+            else:
+                try:
+                    cause = node.cause.infer().next()
+                except astroid.InferenceError:
+                    pass
+                else:
+                    if (not isinstance(cause, astroid.Class) and
+                        not inherit_from_std_ex(cause)):
+                        self.add_message('bad-exception-context',
+                                         node=node)
         expr = node.exc
         if self._check_raise_value(node, expr):
             return
diff --git a/test/input/func_bad_exception_context_py30.py b/test/input/func_bad_exception_context_py30.py
new file mode 100644
index 0000000..b520e68
--- /dev/null
+++ b/test/input/func_bad_exception_context_py30.py
@@ -0,0 +1,17 @@
+"""Check that raise ... from .. uses a proper exception context """
+
+# pylint: disable=unreachable
+
+__revision__ = 0
+
+class ExceptionSubclass(Exception):
+    """ subclass """
+
+def test():
+    """ docstring """
+    raise IndexError from 1
+    raise IndexError from None
+    raise IndexError from ZeroDivisionError
+    raise IndexError from object()
+    raise IndexError from ExceptionSubclass
+
diff --git a/test/messages/func_bad_exception_context_py30.txt b/test/messages/func_bad_exception_context_py30.txt
new file mode 100644
index 0000000..0a4b3ec
--- /dev/null
+++ b/test/messages/func_bad_exception_context_py30.txt
@@ -0,0 +1,2 @@
+E: 12:test: Exception context set to something which is not an exception, nor None

+E: 15:test: Exception context set to something which is not an exception, nor None
\ No newline at end of file