New check: bad-len-returned

Implementation of issue #557.

pylint/checkers/classes.py

@@ -995,16 +995,22 @@ class SpecialMethodsChecker(BaseChecker):
                   'invalid number of parameters. If it has too few or '
                   'too many, it might not work at all.',
                   {'old_names': [('E0235', 'bad-context-manager')]}),
+        'E0303': ('__len__ does not return non-negative integer',
+                  'bad-len-returned',
+                  'Used when an __len__ method returns something which is not a '
+                  'non-negative integer', {}),
     }
     priority = -2
 
     @check_messages('unexpected-special-method-signature',
-                    'non-iterator-returned')
+                    'non-iterator-returned', 'bad-len-returned')
     def visit_functiondef(self, node):
         if not node.is_method():
             return
         if node.name == '__iter__':
             self._check_iter(node)
+        if node.name == '__len__':
+            self._check_len(node)
         if node.name in PYMETHODS:
             self._check_unexpected_method_signature(node)
 
@@ -1086,6 +1092,14 @@ def _check_iter(self, node):
             if not self._is_iterator(infered):
                 self.add_message('non-iterator-returned', node=node)
 
+    def _check_len(self, node):
+        infered = _safe_infer_call_result(node, node)
+        if infered is None or infered is astroid.util.Uninferable:
+            return
+        value = infered.value
+        if not isinstance(value, six.integer_types) or not value >= 0:
+            self.add_message('bad-len-returned', node=node)
+
 
 def _ancestors_to_call(klass_node, method='__init__'):
     """return a dictionary where keys are the list of base classes providing

pylint/test/input/func_e0303.py

@@ -0,0 +1,44 @@
+"""Check bad value returned by __len__ """
+
+# pylint: disable=too-few-public-methods, missing-docstring, no-self-use
+import sys
+
+import six
+
+
+class FirstGoodLen(object):
+    """__len__ returns <type 'int'>"""
+
+    def __len__(self):
+        return 0
+
+
+class SecondGoodLen(object):
+    """__len__ returns <type 'long'>"""
+
+    def __len__(self):
+        return sys.maxsize + 1
+
+
+class LenMetaclass(type):
+    def __len__(cls):
+        return 1
+
+
+@six.add_metaclass(LenMetaclass)
+class ThirdGoodLen(object):
+    """Length through the metaclass."""
+
+
+class FirstBadLen(object):
+    """ __len__ returns a negative integer """
+
+    def __len__(self):
+        return -1
+
+
+class SecondBadLength(object):
+    """ __len__ float """
+
+    def __len__(self):
+        return 3.0

pylint/test/messages/func_e0303.txt

@@ -0,0 +1,2 @@
+E: 36:FirstBadLen.__len__: __len__ does not return non-negative integer
+E: 43:SecondBadLength.__len__: __len__ does not return non-negative integer