python: port py/comparison-using-is

see triage [here](github/codeql-python-team#628 (comment))
- no longer try to interpret the class of operands
- simply alert in clear bad cases of uninterned literals
- surprisingly(?), all tests still pass

Author: yoff

python/ql/src/Expressions/IncorrectComparisonUsingIs.ql

@@ -11,16 +11,57 @@
  */
 
 import python
-import IsComparisons
 
-from Compare comp, Cmpop op, ClassValue c, string alt
-where
-  invalid_portable_is_comparison(comp, op, c) and
-  not cpython_interned_constant(comp.getASubExpression()) and
+/** Holds if the comparison `comp` uses `is` or `is not` (represented as `op`) to compare its `left` and `right` arguments. */
+predicate comparison_using_is(Compare comp, ControlFlowNode left, Cmpop op, ControlFlowNode right) {
+  exists(CompareNode fcomp | fcomp = comp.getAFlowNode() |
+    fcomp.operands(left, op, right) and
+    (op instanceof Is or op instanceof IsNot)
+  )
+}
+
+private predicate cpython_interned_value(Expr e) {
+  exists(string text | text = e.(StrConst).getText() |
+    text.length() = 0
+    or
+    text.length() = 1 and text.regexpMatch("[U+0000-U+00ff]")
+  )
+  or
+  exists(int i | i = e.(IntegerLiteral).getN().toInt() | -5 <= i and i <= 256)
+  or
+  exists(Tuple t | t = e and not exists(t.getAnElt()))
+}
+
+predicate uninterned_literal(Expr e) {
   (
-    op instanceof Is and alt = "=="
+    e instanceof StrConst
+    or
+    e instanceof IntegerLiteral
+    or
+    e instanceof FloatLiteral
+    or
+    e instanceof Dict
+    or
+    e instanceof List
+    or
+    e instanceof Tuple
+  ) and
+  not cpython_interned_value(e)
+}
+
+from Compare comp, Cmpop op, string alt
+where
+  exists(ControlFlowNode left, ControlFlowNode right |
+    comparison_using_is(comp, left, op, right) and
+    (
+      op instanceof Is and alt = "=="
+      or
+      op instanceof IsNot and alt = "!="
+    )
+  |
+    uninterned_literal(left.getNode())
     or
-    op instanceof IsNot and alt = "!="
+    uninterned_literal(right.getNode())
   )
 select comp,
   "Values compared using '" + op.getSymbol() +