Check that the argument passed to reversed() is a proper sequence.

1 files changed, 38 insertions, 1 deletions

diff --git a/checkers/base.py b/checkers/base.py
index de40c4c..12bc8b3 100644
--- a/checkers/base.py
+++ b/checkers/base.py
@@ -33,6 +33,8 @@ from pylint.checkers.utils import (
     is_inside_except,
     overrides_a_method,
     safe_infer,
+    get_argument_from_call,
+    NoSuchArgumentError,
     )
 
 
@@ -47,6 +49,8 @@ DEFAULT_NAME_RGX = re.compile('[a-z_][a-z0-9_]{2,30}$')
 CLASS_ATTRIBUTE_RGX = re.compile(r'([A-Za-z_][A-Za-z0-9_]{2,30}|(__.*__))$')
 # do not require a doc string on system methods
 NO_REQUIRED_DOC_RGX = re.compile('__.*__')
+REVERSED_METHODS = (('__getitem__', '__len__'),
+                    ('__reversed__', ))
 
 del re
 
@@ -409,6 +413,16 @@ functions, methods
     'C0121': ('Missing required attribute "%s"', # W0103
               'missing-module-attribute',
               'Used when an attribute required for modules is missing.'),
+
+    'E0109': ('Missing argument to reversed()',
+              'missing-reversed-argument',
+              'Used when reversed() builtin didn\'t receive an argument.'),
+    'E0111': ('The first reversed() argument is not a sequence',
+              'bad-reversed-sequence',
+              'Used when the first argument to reversed() builtin '
+              'isn\'t a sequence (does not implement __reversed__, '
+              'nor __getitem__ and __len__'),
+
     }
 
     options = (('required-attributes',
@@ -598,7 +612,8 @@ functions, methods
         """just print a warning on exec statements"""
         self.add_message('exec-used', node=node)
 
-    @check_messages('bad-builtin', 'star-args', 'exec-used')
+    @check_messages('bad-builtin', 'star-args', 
+                    'exec-used', 'missing-reversed-argument')
     def visit_callfunc(self, node):
         """visit a CallFunc node -> check if this is not a blacklisted builtin
         call and check for * or ** use
@@ -611,6 +626,8 @@ functions, methods
                     name in node.root()):
                 if name == 'exec':
                     self.add_message('exec-used', node=node)
+                elif name == 'reversed':
+                    self._check_reversed(node)
                 if name in self.config.bad_functions:
                     self.add_message('bad-builtin', node=node, args=name)
         if node.starargs or node.kwargs:
@@ -675,7 +692,27 @@ functions, methods
                 return
             _node = _parent
             _parent = _node.parent
+    
+    def _check_reversed(self, node):
+        """ check that the argument to `reversed` is a sequence """
+        try:
+            argument = safe_infer(get_argument_from_call(node, position=0))
+        except NoSuchArgumentError:
+            self.add_message('missing-reversed-argument', node=node)
+        else:
+            if not argument:
+                return
 
+            for methods in REVERSED_METHODS:
+                for meth in methods:
+                    try:
+                        argument.getattr(meth)
+                    except astroid.NotFoundError:
+                        break
+                else:
+                    break
+            else:                 
+                self.add_message('bad-reversed-sequence', node=node)
 
 class NameChecker(_BasicChecker):
     msgs = {