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

It is emitted when item deletion is tried on an object which doesn't
have this ability. Closes issue #592.

2 files changed, 111 insertions, 0 deletions

diff --git a/pylint/test/functional/unsupported_delete_operation.py b/pylint/test/functional/unsupported_delete_operation.py
new file mode 100644
index 0000000..297cc15
--- /dev/null
+++ b/pylint/test/functional/unsupported_delete_operation.py
@@ -0,0 +1,94 @@
+"""
+Checks that value used in a subscript support deletion
+(i.e. defines __delitem__ 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]
+del numbers[0]
+
+
+del bytearray(b"123")[0]
+del dict(a=1, b=2)['a']
+del (1, 2, 3)[0] # [unsupported-delete-operation]
+
+# list/dict comprehensions are fine
+del [x for x in range(10)][0]
+del {x: 10 - x for x in range(10)}[0]
+
+
+# instances
+class NonSubscriptable(object):
+    pass
+
+class Subscriptable(object):
+    def __delitem__(self, key):
+        pass
+
+del NonSubscriptable()[0]  # [unsupported-delete-operation]
+del NonSubscriptable[0] # [unsupported-delete-operation]
+del Subscriptable()[0]
+del Subscriptable[0] # [unsupported-delete-operation]
+
+# generators are not subscriptable
+def powers_of_two():
+    k = 0
+    while k < 10:
+        yield 2 ** k
+        k += 1
+
+del powers_of_two()[0] # [unsupported-delete-operation]
+del powers_of_two[0]  # [unsupported-delete-operation]
+
+
+# check that primitive non subscriptable types are catched
+del True[0]  # [unsupported-delete-operation]
+del None[0] # [unsupported-delete-operation]
+del 8.5[0] # [unsupported-delete-operation]
+del 10[0] # [unsupported-delete-operation]
+
+# sets are not subscriptable
+del {x ** 2 for x in range(10)}[0] # [unsupported-delete-operation]
+del set(numbers)[0] # [unsupported-delete-operation]
+del frozenset(numbers)[0] # [unsupported-delete-operation]
+
+# skip instances with unknown base classes
+from some_missing_module import LibSubscriptable
+
+class MaybeSubscriptable(LibSubscriptable):
+    pass
+
+del MaybeSubscriptable()[0]
+
+# subscriptable classes (through metaclasses)
+
+class MetaSubscriptable(type):
+    def __delitem__(cls, key):
+        pass
+
+class SubscriptableClass(six.with_metaclass(MetaSubscriptable, object)):
+    pass
+
+del SubscriptableClass[0]
+del SubscriptableClass()[0] # [unsupported-delete-operation]
+
+# functions are not subscriptable
+def test(*args, **kwargs):
+    return args, kwargs
+
+del test()[0] # [unsupported-delete-operation]
+del test[0] # [unsupported-delete-operation]
+
+# deque
+from collections import deque
+deq = deque(maxlen=10)
+deq.append(42)
+del deq[0]
+
+# tuples assignment
+values = [1, 2, 3, 4]
+del (values[0], values[1])
+del (values[0], SubscriptableClass()[0]) # [unsupported-delete-operation]
diff --git a/pylint/test/functional/unsupported_delete_operation.txt b/pylint/test/functional/unsupported_delete_operation.txt
new file mode 100644
index 0000000..2eead9b
--- /dev/null
+++ b/pylint/test/functional/unsupported_delete_operation.txt
@@ -0,0 +1,17 @@
+unsupported-delete-operation:16::'(1, 2, 3)' does not support item deletion
+unsupported-delete-operation:31::'NonSubscriptable()' does not support item deletion
+unsupported-delete-operation:32::'NonSubscriptable' does not support item deletion
+unsupported-delete-operation:34::'Subscriptable' does not support item deletion
+unsupported-delete-operation:43::'powers_of_two()' does not support item deletion
+unsupported-delete-operation:44::'powers_of_two' does not support item deletion
+unsupported-delete-operation:48::'True' does not support item deletion
+unsupported-delete-operation:49::'None' does not support item deletion
+unsupported-delete-operation:50::'8.5' does not support item deletion
+unsupported-delete-operation:51::'10' does not support item deletion
+unsupported-delete-operation:54::'{(x) ** (2) for x in range(10)}' does not support item deletion
+unsupported-delete-operation:55::'set(numbers)' does not support item deletion
+unsupported-delete-operation:56::'frozenset(numbers)' does not support item deletion
+unsupported-delete-operation:76::'SubscriptableClass()' does not support item deletion
+unsupported-delete-operation:82::'test()' does not support item deletion
+unsupported-delete-operation:83::'test' does not support item deletion
+unsupported-delete-operation:94::'SubscriptableClass()' does not support item deletion