Initial commit of Python queries and QL libraries.

python/ql/src/Classes/MutatingDescriptor.ql

@@ -0,0 +1,28 @@
+/**
+ * @name Mutation of descriptor in __get__ or __set__ method.
+ * @description Descriptor objects can be shared across many instances. Mutating them can cause strange side effects or race conditions.
+ * @kind problem
+ * @tags reliability
+ *       correctness
+ * @problem.severity error
+ * @sub-severity low
+ * @precision very-high
+ * @id py/mutable-descriptor
+ */
+
+import python
+
+predicate mutates_descriptor(ClassObject cls, SelfAttributeStore s) {
+    cls.isDescriptorType() and
+    exists(PyFunctionObject f |
+        cls.lookupAttribute(_) = f and 
+        not f.getName() = "__init__" and
+        s.getScope() = f.getFunction()
+    )
+}
+
+from ClassObject cls, SelfAttributeStore s
+where
+mutates_descriptor(cls, s)
+
+select s, "Mutation of descriptor $@ object may lead to action-at-a-distance effects or race conditions for properties.", cls, cls.getName()