First attempt at centering images and QRs (#250)

This was tested on ZJ-5890 with success. By default centering is
deactivated for backward compatibility. Trying to center a QR code in
native mode will raise an exception as we do not know ATM if the native
rendering is centered by default or not.

* Added basic tests for center feature

* Check image size before centering

test/test_function_image.py

@@ -145,10 +145,8 @@ def test_large_graphics():
     assert(instance.output == b'\x1dv0\x00\x01\x00\x01\x00\xc0\x1dv0\x00\x01\x00\x01\x00\x00')
 
 
-def test_width_too_large():
-    """
-    Test printing an image that is too large in width.
-    """
+@pytest.fixture
+def dummy_with_width():
     instance = printer.Dummy()
     instance.profile.profile_data = {
         'media': {
@@ -157,8 +155,25 @@ def test_width_too_large():
             }
         }
     }
+    return instance
+
+
+def test_width_too_large(dummy_with_width):
+    """
+    Test printing an image that is too large in width.
+    """
+    instance = dummy_with_width
 
     with pytest.raises(ImageWidthError):
         instance.image(Image.new("RGB", (385, 200)))
 
-    instance.image(Image.new("RGB", (384, 200)))
+    instance.image(Image.new("RGB", (384, 200)))
+
+
+def test_center_image(dummy_with_width):
+    instance = dummy_with_width
+
+    with pytest.raises(ImageWidthError):
+        instance.image(Image.new("RGB", (385, 200)), center=True)
+
+    instance.image(Image.new("RGB", (384, 200)), center=True)

test/test_function_qr_native.py

@@ -13,6 +13,8 @@ from __future__ import print_function
 from __future__ import unicode_literals
 
 from nose.tools import raises
+import pytest
+
 import escpos.printer as printer
 from escpos.constants import QR_ECLEVEL_H, QR_MODEL_1
 
@@ -25,7 +27,6 @@ def test_defaults():
         b'(k\x07\x001P01234\x1d(k\x03\x001Q0'
     assert(instance.output == expected)
 
-
 def test_empty():
     """Test QR printing blank code"""
     instance = printer.Dummy()
@@ -99,3 +100,13 @@ def test_image_invalid_model():
     """Test unsupported QR model as image"""
     instance = printer.Dummy()
     instance.qr("1234", native=False, model=QR_MODEL_1)
+
+
+@pytest.fixture
+def instance():
+    return printer.Dummy()
+
+
+def test_center_not_implementer(instance):
+    with pytest.raises(NotImplementedError):
+        instance.qr("test", center=True, native=True)

test/test_function_qr_non-native.py

@@ -13,6 +13,7 @@ from __future__ import division
 from __future__ import print_function
 from __future__ import unicode_literals
 
+import pytest
 import mock
 
 from escpos.printer import Dummy
@@ -30,3 +31,12 @@ def test_type_of_object_passed_to_image_function(img_function):
     d.qr("LoremIpsum")
     args, kwargs = img_function.call_args
     assert isinstance(args[0], Image.Image)
+
+
+@pytest.fixture
+def instance():
+    return Dummy()
+
+
+def test_center(instance):
+    instance.qr("LoremIpsum", center=True)