Use nested lists instead of flat ones for Matrix initialization

Input looks like:
[[a, b],
 [c, d]]

instead of [a, b, c, d].

cadquery/occ_impl/geom.py

@@ -161,6 +161,18 @@ class Matrix:
     """A 3d , 4x4 transformation matrix.
 
     Used to move geometry in space.
+
+    The provided "matrix" parameter may be None, a gp_Trsf, or a nested list of
+    values.
+
+    If given a nested list, it is expected to be of the form:
+
+        [[m11, m12, m13, m14],
+         [m21, m22, m23, m24],
+         [m31, m32, m33, m34]]
+
+    A fourth row may be given, but it is expected to be: [0.0, 0.0, 0.0, 1.0]
+    since this is a transform matrix.
     """
 
     def __init__(self, matrix=None):
@@ -171,14 +183,16 @@ class Matrix:
             self.wrapped = matrix
         elif isinstance(matrix, list):
             self.wrapped = gp_Trsf()
-            if len(matrix) == 12:
-                self.wrapped.SetValues(*matrix)
-            elif len(matrix) == 16:
-                # Only use first 12 values - the last 4 must be [0, 0, 0, 1].
-                last4 = matrix[12:]
-                if last4 != [0., 0., 0., 1.]:
-                    raise ValueError("Expected the last 4 values to be [0,0,0,1], but got: {}".format(last4))
-                self.wrapped.SetValues(*matrix[0:12])
+            if len(matrix) == 3:
+                flattened = [e for row in matrix for e in row]
+                self.wrapped.SetValues(*flattened)
+            elif len(matrix) == 4:
+                # Only use first 3 rows - the last must be [0, 0, 0, 1].
+                lastRow = matrix[3]
+                if lastRow != [0., 0., 0., 1.]:
+                    raise ValueError("Expected the last row to be [0,0,0,1], but got: {}".format(lastRow))
+                flattened = [e for row in matrix[0:3] for e in row]
+                self.wrapped.SetValues(*flattened)
             else:
                 raise TypeError("Matrix constructor requires list of length 12 or 16")
         else:

tests/TestCadObjects.py

@@ -117,19 +117,19 @@ class TestCadObjects(BaseTest):
 
     def testMatrixCreationAndAccess(self):
         def matrix_vals(m):
-            return [m[r,c] for r in range(4) for c in range(4)]
+            return [[m[r,c] for c in range(4)] for r in range(4)]
         # default constructor creates a 4x4 identity matrix
         m = Matrix()
-        identity = [1., 0., 0., 0.,
-                    0., 1., 0., 0.,
-                    0., 0., 1., 0.,
-                    0., 0., 0., 1.]
+        identity = [[1., 0., 0., 0.],
+                    [0., 1., 0., 0.],
+                    [0., 0., 1., 0.],
+                    [0., 0., 0., 1.]]
         self.assertEqual(identity, matrix_vals(m))
 
-        vals4x4 = [1., 0., 0., 1.,
-                   0., 1., 0., 2.,
-                   0., 0., 1., 3.,
-                   0., 0., 0., 1.,]
+        vals4x4 = [[1., 0., 0., 1.],
+                   [0., 1., 0., 2.],
+                   [0., 0., 1., 3.],
+                   [0., 0., 0., 1.]]
 
         # test constructor with 16-value input
         m = Matrix(vals4x4)
@@ -141,10 +141,10 @@ class TestCadObjects(BaseTest):
         self.assertEqual(vals4x4, matrix_vals(m))
 
         # Test 16-value input with invalid values for the last 4
-        invalid = [1., 0., 0., 1.,
-                   0., 1., 0., 2.,
-                   0., 0., 1., 3.,
-                   1., 2., 3., 4.,]
+        invalid = [[1., 0., 0., 1.],
+                   [0., 1., 0., 2.],
+                   [0., 0., 1., 3.],
+                   [1., 2., 3., 4.]]
         with self.assertRaises(ValueError):
             Matrix(invalid)