Fix a bad-reversed-sequence false positive.

This patch looks for a __reversed__ method for subclasses
of dict (this is especially present in OrderedDict implementations).

3 files changed, 25 insertions, 5 deletions

diff --git a/ChangeLog b/ChangeLog
index 4add9d5..dab9381 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -47,6 +47,9 @@ ChangeLog for Pylint
 
     * Don't warn about no-self-use for builtin properties.
 
+    * Fix a false positive for bad-reversed-sequence, when a subclass
+      of a `dict` provides a __reversed__ method.
+
 
 2015-03-14 -- 1.4.3
 
diff --git a/pylint/checkers/base.py b/pylint/checkers/base.py
index 7659fa4..6b9cdac 100644
--- a/pylint/checkers/base.py
+++ b/pylint/checkers/base.py
@@ -58,8 +58,10 @@ 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__', ))
+REVERSED_PROTOCOL_METHOD = '__reversed__'
+SEQUENCE_PROTOCOL_METHODS = ('__getitem__', '__len__')
+REVERSED_METHODS = (SEQUENCE_PROTOCOL_METHODS,
+                    (REVERSED_PROTOCOL_METHOD, ))
 
 PY33 = sys.version_info >= (3, 3)
 PY3K = sys.version_info >= (3, 0)
@@ -856,8 +858,12 @@ functions, methods
                     return
                 elif any(ancestor.name == 'dict' and is_builtin_object(ancestor)
                          for ancestor in argument._proxied.ancestors()):
-                    # mappings aren't accepted by reversed()
-                    self.add_message('bad-reversed-sequence', node=node)
+                    # Mappings aren't accepted by reversed(), unless
+                    # they provide explicitly a __reversed__ method.
+                    try:
+                        argument.getattr(REVERSED_PROTOCOL_METHOD)
+                    except astroid.NotFoundError:
+                        self.add_message('bad-reversed-sequence', node=node)
                     return
 
                 for methods in REVERSED_METHODS:
diff --git a/pylint/test/functional/bad_reversed_sequence.py b/pylint/test/functional/bad_reversed_sequence.py
index 6fda2c0..852510f 100644
--- a/pylint/test/functional/bad_reversed_sequence.py
+++ b/pylint/test/functional/bad_reversed_sequence.py
@@ -1,5 +1,5 @@
 """ Checks that reversed() receive proper argument """
-
+# pylint: disable=missing-docstring
 # pylint: disable=too-few-public-methods,no-self-use,no-absolute-import
 from collections import deque
 
@@ -58,3 +58,14 @@ def test(path):
     seq = uninferable([1, 2, 3])
     seq = reversed(path.split("/"))
     return seq
+
+def test_dict_ancestor_and_reversed():
+    """Don't emit for subclasses of dict, with __reversed__ implemented."""
+    from collections import OrderedDict
+
+    class Child(dict):
+        def __reversed__(self):
+            return reversed(range(10))
+
+    seq = reversed(OrderedDict())
+    return reversed(Child()), seq