558 improve capabilities handling (#562)

* add handling for missing capabilities file

* improve documentation

* Update doc/user/installation.rst

test/test_load_module_missing_capability.py

@@ -0,0 +1,29 @@
+#!/usr/bin/python
+"""basic test case that simulates an empty capability file
+
+:author: `Patrick Kanzler <dev@pkanzler.de>`_
+:organization: `python-escpos <https://github.com/python-escpos>`_
+:copyright: Copyright (c) 2016 `python-escpos <https://github.com/python-escpos>`_
+:license: MIT
+"""
+
+import os
+import tempfile
+
+import pytest
+
+
+def test_instantiation():
+    """test the instantiation of a escpos-printer class"""
+    # inject an environment variable that points to an empty capabilities file
+    os.environ["ESCPOS_CAPABILITIES_FILE"] = tempfile.NamedTemporaryFile().name
+
+    import escpos.printer as printer
+    from escpos.exceptions import BarcodeTypeError
+
+    # remove again the variable (so that no other tests are affected)
+    os.environ.pop("ESCPOS_CAPABILITIES_FILE")
+
+    instance = printer.Dummy()
+    with pytest.raises(BarcodeTypeError):
+        instance.barcode("bc", "code")