Added a new warning, 'unsupported-assignment-operation'

This is emitted when item assignment is tried on an object which doesn't
have this ability. Closes issue #591.

5 files changed, 162 insertions, 9 deletions

diff --git a/ChangeLog b/ChangeLog
index 2436dcc..f36f175 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,10 @@ ChangeLog for Pylint
 --------------------
 
 --
+    * Added a new warning, 'unsupported-assignment-operation', which is
+      emitted when item assignment is tried on an object which doesn't
+      have this ability. Closes issue #591.
+
     * Added multiple warnings related to imports. 'wrong-import-order'
       is emitted when PEP 8 recommendations regarding imports are not
       respected (that is, standard imports should be followed by third-party
diff --git a/pylint/checkers/typecheck.py b/pylint/checkers/typecheck.py
index 43b0bde..75cb193 100644
--- a/pylint/checkers/typecheck.py
+++ b/pylint/checkers/typecheck.py
@@ -38,7 +38,8 @@ from pylint.checkers.utils import (
     decorated_with, node_ignores_exception,
     is_iterable, is_mapping, supports_membership_test,
     is_comprehension, is_inside_abstract_class,
-    supports_subscript,
+    supports_getitem,
+    supports_setitem,
     safe_infer,
     has_known_bases)
 from pylint import utils
@@ -161,6 +162,10 @@ MSGS = {
               'unsubscriptable-object',
               "Emitted when a subscripted value doesn't support subscription"
               "(i.e. doesn't define __getitem__ method)"),
+    'E1137': ("%r does not support item assignment",
+              'unsupported-assignment-operation',
+              "Emitted when an object does not support item assignment "
+              "(i.e. doesn't define __setitem__ method)"),
     }
 
 # builtin sequence types in Python 2 and 3.
@@ -825,18 +830,42 @@ accessed. Python regular expressions are accepted.'}
         if operator in ['in', 'not in']:
             self._check_membership_test(right)
 
+    @staticmethod
+    def _is_subscript_store_context(node):
+        statement = node.statement()
+        if isinstance(statement, astroid.Assign):
+            for target in statement.targets:
+                if target is node or target.parent_of(node):
+                    return False
+        return True
+
     @check_messages('unsubscriptable-object')
     def visit_subscript(self, node):
         if isinstance(node.value, (astroid.ListComp, astroid.DictComp)):
             return
+
+        store_context = self._is_subscript_store_context(node)
         if isinstance(node.value, astroid.SetComp):
-            self.add_message('unsubscriptable-object',
-                             args=node.value.as_string(),
+            if store_context:
+                msg = 'unsubscriptable-object'
+            else:
+                msg = 'unsupported-assignment-operation'
+            self.add_message(msg, args=node.value.as_string(),
                              node=node.value)
-        infered = safe_infer(node.value)
-        if infered is None or infered is astroid.YES:
             return
-        if not supports_subscript(infered):
+
+        inferred = safe_infer(node.value)
+        if inferred is None or inferred is astroid.YES:
+            return
+
+        if not store_context:
+            if not supports_setitem(inferred):
+                self.add_message('unsupported-assignment-operation',
+                                 args=node.value.as_string(),
+                                 node=node.value)
+            return
+
+        if not supports_getitem(inferred):
             self.add_message('unsubscriptable-object',
                              args=node.value.as_string(),
                              node=node.value)
diff --git a/pylint/checkers/utils.py b/pylint/checkers/utils.py
index e24a034..ba396b0 100644
--- a/pylint/checkers/utils.py
+++ b/pylint/checkers/utils.py
@@ -43,6 +43,7 @@ ABC_METHODS = set(('abc.abstractproperty', 'abc.abstractmethod',
 ITER_METHOD = '__iter__'
 NEXT_METHOD = 'next' if six.PY2 else '__next__'
 GETITEM_METHOD = '__getitem__'
+SETITEM_METHOD = '__setitem__'
 CONTAINS_METHOD = '__contains__'
 KEYS_METHOD = 'keys'
 
@@ -618,10 +619,14 @@ def _supports_iteration_protocol(value):
     return _hasattr(value, ITER_METHOD) or _hasattr(value, GETITEM_METHOD)
 
 
-def _supports_subscript_protocol(value):
+def _supports_getitem_protocol(value):
     return _hasattr(value, GETITEM_METHOD)
 
 
+def _supports_setitem_protocol(value):
+    return _hasattr(value, SETITEM_METHOD)
+
+
 def _is_abstract_class_name(name):
     lname = name.lower()
     is_mixin = lname.endswith('mixin')
@@ -672,8 +677,12 @@ def supports_membership_test(value):
     return supported or is_iterable(value)
 
 
-def supports_subscript(value):
-    return _supports_protocol(value, _supports_subscript_protocol)
+def supports_getitem(value):
+    return _supports_protocol(value, _supports_getitem_protocol)
+
+
+def supports_setitem(value):
+    return _supports_protocol(value, _supports_setitem_protocol)
 
 
 # TODO(cpopa): deprecate these or leave them as aliases?
diff --git a/pylint/test/functional/unsupported_assignment_operation.py b/pylint/test/functional/unsupported_assignment_operation.py
new file mode 100644
index 0000000..a2e5f87
--- /dev/null
+++ b/pylint/test/functional/unsupported_assignment_operation.py
@@ -0,0 +1,94 @@
+"""
+Checks that value used in a subscript support assignments
+(i.e. defines __setitem__ method).
+"""
+# pylint: disable=missing-docstring,pointless-statement,expression-not-assigned,wrong-import-position
+# pylint: disable=too-few-public-methods,import-error,invalid-name,wrong-import-order
+import six
+
+# primitives
+numbers = [1, 2, 3]
+numbers[0] = 42
+
+
+bytearray(b"123")[0] = 42
+dict(a=1, b=2)['a'] = 42
+(1, 2, 3)[0] = 42 # [unsupported-assignment-operation]
+
+# list/dict comprehensions are fine
+[x for x in range(10)][0] = 42
+{x: 10 - x for x in range(10)}[0] = 42
+
+
+# instances
+class NonSubscriptable(object):
+    pass
+
+class Subscriptable(object):
+    def __setitem__(self, key, value):
+        return key + value
+
+NonSubscriptable()[0] = 24  # [unsupported-assignment-operation]
+NonSubscriptable[0] = 24 # [unsupported-assignment-operation]
+Subscriptable()[0] = 24
+Subscriptable[0] = 24 # [unsupported-assignment-operation]
+
+# generators are not subscriptable
+def powers_of_two():
+    k = 0
+    while k < 10:
+        yield 2 ** k
+        k += 1
+
+powers_of_two()[0] = 42 # [unsupported-assignment-operation]
+powers_of_two[0] = 42  # [unsupported-assignment-operation]
+
+
+# check that primitive non subscriptable types are catched
+True[0] = 24  # [unsupported-assignment-operation]
+None[0] = 42 # [unsupported-assignment-operation]
+8.5[0] = 24 # [unsupported-assignment-operation]
+10[0] = 24 # [unsupported-assignment-operation]
+
+# sets are not subscriptable
+{x ** 2 for x in range(10)}[0] = 24 # [unsupported-assignment-operation]
+set(numbers)[0] = 24 # [unsupported-assignment-operation]
+frozenset(numbers)[0] = 42 # [unsupported-assignment-operation]
+
+# skip instances with unknown base classes
+from some_missing_module import LibSubscriptable
+
+class MaybeSubscriptable(LibSubscriptable):
+    pass
+
+MaybeSubscriptable()[0] = 42
+
+# subscriptable classes (through metaclasses)
+
+class MetaSubscriptable(type):
+    def __setitem__(cls, key, value):
+        return key + value
+
+class SubscriptableClass(six.with_metaclass(MetaSubscriptable, object)):
+    pass
+
+SubscriptableClass[0] = 24
+SubscriptableClass()[0] = 24 # [unsupported-assignment-operation]
+
+# functions are not subscriptable
+def test(*args, **kwargs):
+    return args, kwargs
+
+test()[0] = 24 # [unsupported-assignment-operation]
+test[0] = 24 # [unsupported-assignment-operation]
+
+# deque
+from collections import deque
+deq = deque(maxlen=10)
+deq.append(42)
+deq[0] = 42
+
+# tuples assignment
+values = [1, 2, 3, 4]
+(values[0], values[1]) = 3, 4
+(values[0], SubscriptableClass()[0]) = 42, 42 # [unsupported-assignment-operation]
diff --git a/pylint/test/functional/unsupported_assignment_operation.txt b/pylint/test/functional/unsupported_assignment_operation.txt
new file mode 100644
index 0000000..9b8f547
--- /dev/null
+++ b/pylint/test/functional/unsupported_assignment_operation.txt
@@ -0,0 +1,17 @@
+unsupported-assignment-operation:16::'(1, 2, 3)' does not support item assignment
+unsupported-assignment-operation:31::'NonSubscriptable()' does not support item assignment
+unsupported-assignment-operation:32::'NonSubscriptable' does not support item assignment
+unsupported-assignment-operation:34::'Subscriptable' does not support item assignment
+unsupported-assignment-operation:43::'powers_of_two()' does not support item assignment
+unsupported-assignment-operation:44::'powers_of_two' does not support item assignment
+unsupported-assignment-operation:48::'True' does not support item assignment
+unsupported-assignment-operation:49::'None' does not support item assignment
+unsupported-assignment-operation:50::'8.5' does not support item assignment
+unsupported-assignment-operation:51::'10' does not support item assignment
+unsupported-assignment-operation:54::'{(x) ** (2) for x in range(10)}' does not support item assignment
+unsupported-assignment-operation:55::'set(numbers)' does not support item assignment
+unsupported-assignment-operation:56::'frozenset(numbers)' does not support item assignment
+unsupported-assignment-operation:76::'SubscriptableClass()' does not support item assignment
+unsupported-assignment-operation:82::'test()' does not support item assignment
+unsupported-assignment-operation:83::'test' does not support item assignment
+unsupported-assignment-operation:94::'SubscriptableClass()' does not support item assignment