Python: Modernise the py/non-iterable-in-for-loop query.

Also adds a small test case exhibiting the same false positive seen in
ODASA-8042.

python/ql/src/Statements/NonIteratorInForLoop.ql

@@ -13,11 +13,11 @@
 
 import python
 
-from For loop, ControlFlowNode iter, ClassObject t, ControlFlowNode origin
+from For loop, ControlFlowNode iter, Value v, ClassValue t, ControlFlowNode origin
 where loop.getIter().getAFlowNode() = iter and
-iter.refersTo(_, t, origin) and 
-not t.isIterable() and not t.failedInference() and
-not t = theNoneType() and
+iter.pointsTo(_, v, origin) and v.getClass() = t and
+not t.isIterable() and not t.failedInference(_) and
+not v = Value::named("None") and
 not t.isDescriptorType()
 
 select loop, "$@ of class '$@' may be used in for-loop.", origin, "Non-iterator", t, t.getName()

python/ql/src/semmle/python/objects/ObjectAPI.qll

@@ -346,6 +346,18 @@ class ClassValue extends Value {
         this.(ClassObjectInternal).lookup("__call__", _, _)
     }
 
+    /** Holds if this class is an iterable. */
+    predicate isIterable() {
+        this.hasAttribute("__iter__")
+        or
+        this.hasAttribute("__getitem__")
+    }
+
+    /** Holds if this class is a descriptor. */
+    predicate isDescriptorType() {
+        this.hasAttribute("__get__")
+    }
+
     /** Gets the qualified name for this class.
      * Should return the same name as the `__qualname__` attribute on classes in Python 3.
      */

python/ql/test/query-tests/Statements/general/NonIteratorInForLoop.expected

@@ -1 +1,2 @@
 | test.py:50:1:50:23 | For | $@ of class '$@' may be used in for-loop. | test.py:50:10:50:22 | ControlFlowNode for NonIterator() | Non-iterator | test.py:45:1:45:26 | class NonIterator | NonIterator |
+| test.py:170:10:170:22 | For | $@ of class '$@' may be used in for-loop. | test.py:170:10:170:22 | ControlFlowNode for .0 | Non-iterator | test.py:169:1:169:21 | class false_positive | false_positive |

python/ql/test/query-tests/Statements/general/test.py

@@ -165,4 +165,7 @@ def no_with():
 def assert_ok(seq):
     assert all(isinstance(element, (str, unicode)) for element in seq)
 
+# False positive. ODASA-8042
+class false_positive:
+    e = (x for x in [])