      "\n",
      "    def test_unique_chars(self, func):\n",
      "        assert_equal(func(''), True)\n",
      "        assert_equal(func('foo'), False)\n",
      "        assert_equal(func('bar'), True)\n",
      "        print('Success: test_unique_chars')\n",
      "\n",
 
</s> remove     "* Can you assume the string is ASCII?\n",
 </s> add     "* Can we assume the string is ASCII?\n", </s> remove     "* Can you assume the string is ASCII?\n",
 </s> add     "* Can we assume the string is ASCII?\n",
    "        assert_equal(func(None), False)\n",
      "\n",
      "    def test_unique_chars(self, func):\n",
    "        assert_equal(func(None), False)\n",
      "        assert_equal(func(''), True)\n",
      "        assert_equal(func('foo'), False)\n",
      "        assert_equal(func('bar'), True)\n",
      "        print('Success: test_unique_chars')\n",
      "\n",
keep add keep keep keep keep keep
 Polish unique chars challenge and solution

Add None input test case.  Update constraints. 
arrays_strings/unique_chars/unique_chars_solution.ipynb