add test for CashDrawerError

test/test_function_cashdraw.py

@@ -0,0 +1,19 @@
+#!/usr/bin/python
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import escpos.printer as printer
+from escpos.exceptions import CashDrawerError
+import pytest
+
+
+def test_raise_CashDrawerError():
+    """should raise an error if the sequence is invalid.
+    """
+    instance = printer.Dummy()
+    with pytest.raises(CashDrawerError):
+        # call with sequence that is too long
+        instance.cashdraw([1,1,1,1,1,1])
+