Create first test that renders and validates the the rendered PNG is correct.

gerber/render/cairo_backend.py

@@ -292,8 +292,7 @@ class GerberCairoContext(GerberContext):
             self.ctx.paint()
 
     def dump(self, filename):
-        is_svg = filename.lower().endswith(".svg")
-        if is_svg:
+        if filename and filename.lower().endswith(".svg"):
             self.surface.finish()
             self.surface_buffer.flush()
             with open(filename, "w") as f:
@@ -301,7 +300,7 @@ class GerberCairoContext(GerberContext):
                 f.write(self.surface_buffer.read())
                 f.flush()
         else:
-            self.surface.write_to_png(filename)
+            return self.surface.write_to_png(filename)
 
     def dump_svg_str(self):
         self.surface.finish()

gerber/tests/resources/example_two_square_boxes.gbr

@@ -0,0 +1,19 @@
+G04 Ucamco ex. 1: Two square boxes*
+%FSLAX25Y25*%
+%MOMM*%
+%TF.Part,Other*%
+%LPD*%
+%ADD10C,0.010*%
+D10*
+X0Y0D02*
+G01*
+X500000Y0D01*
+Y500000D01*
+X0D01*
+Y0D01*
+X600000D02*
+X1100000D01*
+Y500000D01*
+X600000D01*
+Y0D01*
+M02*

gerber/tests/test_cairo_backend.py

@@ -0,0 +1,59 @@
+#! /usr/bin/env python
+# -*- coding: utf-8 -*-
+
+# Author: Garret Fick <garret@ficksworkshop.com>
+import os
+import io
+
+from ..render.cairo_backend import GerberCairoContext
+from ..rs274x import read, GerberFile
+from .tests import *
+
+
+
+TWO_BOXES_FILE = os.path.join(os.path.dirname(__file__),
+                                'resources/example_two_square_boxes.gbr')
+TWO_BOXES_EXPECTED = os.path.join(os.path.dirname(__file__),
+                                'golden/example_two_square_boxes.png')
+
+def test_render_polygon():
+
+    _test_render(TWO_BOXES_FILE, TWO_BOXES_EXPECTED)
+
+def _test_render(gerber_path, png_expected_path, create_output_path = None):
+    """Render the gerber file and compare to the expected PNG output.
+    
+    Parameters
+    ----------
+    gerber_path : string
+        Path to Gerber file to open
+    png_expected_path : string
+        Path to the PNG file to compare to
+    create_output : string|None
+        If not None, write the generated PNG to the specified path.
+        This is primarily to help with 
+    """
+    
+    gerber = read(gerber_path)
+
+    # Create PNG image to the memory stream
+    ctx = GerberCairoContext()
+    gerber.render(ctx)
+
+    actual_bytes = ctx.dump(None)
+    
+    # If we want to write the file bytes, do it now. This happens
+    if create_output_path:
+        with open(create_output_path, 'wb') as out_file:
+            out_file.write(actual_bytes)
+        # Creating the output is dangerous - it could overwrite the expected result.
+        # So if we are creating the output, we make the test fail on purpose so you
+        # won't forget to disable this
+        assert_false(True, 'Test created the output %s. This needs to be disabled to make sure the test behaves correctly' % (create_output_path,))
+    
+    # Read the expected PNG file
+    
+    with open(png_expected_path, 'rb') as expected_file:
+        expected_bytes = expected_file.read()
+    
+    assert_equal(expected_bytes, actual_bytes)

gerber/tests/test_primitives.py

@@ -9,10 +9,18 @@ from operator import add
 
 def test_primitive_smoketest():
     p = Primitive()
-    assert_raises(NotImplementedError, p.bounding_box)
+    try:
+        p.bounding_box
+        assert_false(True, 'should have thrown the exception')
+    except NotImplementedError: 
+        pass
     p.to_metric()
     p.to_inch()
-    p.offset(1, 1)
+    try:
+        p.offset(1, 1)
+        assert_false(True, 'should have thrown the exception')
+    except NotImplementedError:
+        pass
 
 def test_line_angle():
     """ Test Line primitive angle calculation