/**
 * @name Iterable can be either a string or a sequence
 * @description Iteration over either a string or a sequence in the same loop can cause errors that are hard to find.
 * @kind problem
 * @tags reliability
 *       maintainability
 *       non-local
 * @problem.severity error
 * @sub-severity low
 * @precision high
 * @id py/iteration-string-and-sequence
 */

import python
import semmle.python.filters.Tests

predicate has_string_type(Value v) {
  v.getClass() = ClassValue::str()
  or
  v.getClass() = ClassValue::unicode() and major_version() = 2
}

from
  For loop, ControlFlowNode iter, Value str, Value seq, ControlFlowNode seq_origin,
  ControlFlowNode str_origin
where
  loop.getIter().getAFlowNode() = iter and
  iter.pointsTo(str, str_origin) and
  iter.pointsTo(seq, seq_origin) and
  has_string_type(str) and
  seq.getClass().isIterable() and
  not has_string_type(seq) and
  // suppress occurrences from tests
  not seq_origin.getScope().getScope*() instanceof TestScope and
  not str_origin.getScope().getScope*() instanceof TestScope
select loop,
  "Iteration over $@, of class " + seq.getClass().getName() + ", may also iterate over $@.",
  seq_origin, "sequence", str_origin, "string"