Add check for unhashable dict keys (fixes #586) (#2089)

6 files changed, 45 insertions, 2 deletions

diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 84531db7d..024eb7e06 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -173,6 +173,8 @@ Order doesn't matter (not that much, at least ;)
 
 * Marianna Polatoglou: minor contribution for wildcard import check
 
+* Ben Green: contributor
+
 * Benjamin Freeman: contributor
 
 * Fureigh: contributor
diff --git a/ChangeLog b/ChangeLog
index 1b404ae4c..833d87739 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -4,6 +4,10 @@ Pylint's ChangeLog
 What's New in Pylint 2.0?
 =========================
 
+    * Add `unhashable-dict-key` check.
+
+      Closes #586
+    
     * Don't warn that a global variable is unused if it is defined by an import
 
       Close #1453
diff --git a/doc/whatsnew/2.0.rst b/doc/whatsnew/2.0.rst
index a0313a100..cfa74e55d 100644
--- a/doc/whatsnew/2.0.rst
+++ b/doc/whatsnew/2.0.rst
@@ -115,6 +115,10 @@ New checkers
          some_value = some_call()
          return locals()
 
+  * New `unhashable-dict-key` check added to detect dict lookups using
+  unhashable keys such as lists or dicts.
+
+
 Other Changes
 =============
 
diff --git a/pylint/checkers/typecheck.py b/pylint/checkers/typecheck.py
index b24f7498c..c7acdb67d 100644
--- a/pylint/checkers/typecheck.py
+++ b/pylint/checkers/typecheck.py
@@ -288,6 +288,10 @@ MSGS = {
               'Emitted whenever we can detect that a class is using, '
               'as a metaclass, something which might be invalid for using as '
               'a metaclass.'),
+    'E1140': ("Dict key is unhashable",
+              'unhashable-dict-key',
+              "Emitted when a dict key is not hashable"
+              "(i.e. doesn't define __hash__ method)"),
     'W1113': ('Keyword argument before variable positional arguments list '
               'in the definition of %s function',
               'keyword-arg-before-vararg',
@@ -1241,13 +1245,28 @@ accessed. Python regular expressions are accepted.'}
         if op in ['in', 'not in']:
             self._check_membership_test(right)
 
-    @check_messages('unsubscriptable-object', 'unsupported-assignment-operation',
-                    'unsupported-delete-operation')
+    @check_messages('unsubscriptable-object',
+                    'unsupported-assignment-operation',
+                    'unsupported-delete-operation',
+                    'unhashable-dict-key')
     def visit_subscript(self, node):
         supported_protocol = None
         if isinstance(node.value, (astroid.ListComp, astroid.DictComp)):
             return
 
+        if isinstance(node.value, astroid.Dict):
+            # Assert dict key is hashable
+            inferred = safe_infer(node.slice.value)
+            if inferred is not None:
+                try:
+                    hash_fn = next(inferred.igetattr('__hash__'))
+                except (astroid.InferenceError, TypeError):
+                    pass
+                else:
+                    if getattr(hash_fn, 'value', True) is None:
+                        self.add_message('unhashable-dict-key',
+                                         node=node.value)
+
         if node.ctx == astroid.Load:
             supported_protocol = supports_getitem
             msg = 'unsubscriptable-object'
diff --git a/pylint/test/functional/unhashable_dict_key.py b/pylint/test/functional/unhashable_dict_key.py
new file mode 100644
index 000000000..54b3065a5
--- /dev/null
+++ b/pylint/test/functional/unhashable_dict_key.py
@@ -0,0 +1,11 @@
+# pylint: disable=missing-docstring,expression-not-assigned,too-few-public-methods,pointless-statement
+
+
+class Unhashable(object):
+    __hash__ = list.__hash__
+
+{}[[1, 2, 3]] # [unhashable-dict-key]
+{}[{}] # [unhashable-dict-key]
+{}[Unhashable()] # [unhashable-dict-key]
+{'foo': 'bar'}['foo']
+{'foo': 'bar'}[42]
diff --git a/pylint/test/functional/unhashable_dict_key.txt b/pylint/test/functional/unhashable_dict_key.txt
new file mode 100644
index 000000000..77c3b9287
--- /dev/null
+++ b/pylint/test/functional/unhashable_dict_key.txt
@@ -0,0 +1,3 @@
+unhashable-dict-key:7::Dict key is unhashable
+unhashable-dict-key:8::Dict key is unhashable
+unhashable-dict-key:9::Dict key is unhashable