Added two new Python 3 porting checks, exception-escape and comprehension-escape

These two are emitted whenever pylint detects that a variable defined in the
said blocks is used outside of the given block. On Python 3 these values are deleted.

Author: PCManticore

ChangeLog

@@ -10,6 +10,11 @@ What's New in Pylint 2.0?
 
        Close #1793
 
+    * Added two new Python 3 porting checks, `exception-escape` and `comprehension-escape`
+
+      These two are emitted whenever pylint detects that a variable defined in the
+      said blocks is used outside of the given block. On Python 3 these values are deleted.
+
     * Added a new `deprecated-sys-function`, emitted when accessing removed sys members.
 
     * Added `xreadlines-attribute`, emitted when the `xreadlines()` attribute is accessed.

pylint/checkers/python3.py

@@ -412,6 +412,16 @@ class Python3Checker(checkers.BaseChecker):
                   'deprecated-sys-function',
                   'Used when accessing a field on sys module that has been '
                   'removed in Python 3.',),
+        'W1661': ('Using an exception object that was bound by an except handler',
+                  'exception-escape',
+                  'Emitted when using an exception, that was bound in an except handler, outside '
+                  'of the except handler. On Python 3 these exceptions will be deleted once they get out '
+                  'of the except handler.'),
+        'W1662': ('Using a variable that was bound inside a comprehension',
+                  'comprehension-escape',
+                  'Emitted when using a variable, that was bound in a comprehension handler, outside '
+                  'of the comprehension itself. On Python 3 these variables will be deleted outside of the '
+                  'comprehension.'),
     }
 
     _bad_builtins = frozenset([
@@ -615,12 +625,28 @@ def visit_arguments(self, node):
 
     def visit_name(self, node):
         """Detect when a "bad" built-in is referenced."""
-        found_node = node.lookup(node.name)[0]
+        found_node, located_statements = node.lookup(node.name)
         if _is_builtin(found_node):
             if node.name in self._bad_builtins:
                 message = node.name.lower() + '-builtin'
                 self.add_message(message, node=node)
 
+        if len(located_statements) == 1:
+            assign_statement = located_statements[0].statement()
+            if isinstance(assign_statement, astroid.ExceptHandler):
+                current = node
+                while current and not isinstance(current.parent, astroid.ExceptHandler):
+                    current = current.parent
+
+                if current and isinstance(current.parent, astroid.ExceptHandler):
+                    return
+                self.add_message('exception-escape', node=node)
+
+            if isinstance(assign_statement, (astroid.Expr, astroid.Assign)):
+                if (isinstance(assign_statement.value, astroid.ListComp)
+                        and not assign_statement.parent_of(node)):
+                    self.add_message('comprehension-escape', node=node)
+
     @utils.check_messages('print-statement')
     def visit_print(self, node):
         self.add_message('print-statement', node=node, always_warn=True)

pylint/test/unittest_checker_python3.py

@@ -711,6 +711,41 @@ def test_bad_operator_attribute(self):
         with self.assertAddsMessages(message):
             self.checker.visit_attribute(node)
 
+    def test_comprehension_escape(self):
+        list_comp, set_comp, dict_comp = astroid.extract_node('''
+        [i for i in range(10)]
+        i #@
+        {c for c in range(10)}
+        c #@
+        {j:j for j in range(10)}
+        j #@
+        ''')
+        message = testutils.Message('comprehension-escape', node=list_comp)
+        with self.assertAddsMessages(message):
+            self.checker.visit_name(list_comp)
+
+        for node in (set_comp, dict_comp):
+            with self.assertNoMessages():
+                self.checker.visit_name(node)
+
+    def test_exception_escape(self):
+        bad, good = astroid.extract_node('''
+        try: 1/0
+        except ValueError as exc:
+            pass
+        exc #@
+        try:
+           2/0
+        except (ValueError, TypeError) as exc:
+           exc = 2
+        exc #@
+        ''')
+        message = testutils.Message('exception-escape', node=bad)
+        with self.assertAddsMessages(message):
+            self.checker.visit_name(bad)
+        with self.assertNoMessages():
+            self.checker.visit_name(good)
+
     def test_bad_sys_attribute(self):
         node = astroid.extract_node('''
         import sys