Add STL to Assemblys export formats (#1101)

* Add STL to Assemblys export formats

Turn the assembly into a compound and use Shapes export method to export
the assembly. This way the export function does not need to be
duplicated, and it is what users have been doing manually to get their
objects.

* Add STL to test matrix

Co-authored-by: AU <adam-urbanczyk@users.noreply.github.com>

cadquery/assembly.py

@@ -27,7 +27,7 @@
 
 # type definitions
 AssemblyObjects = Union[Shape, Workplane, None]
-ExportLiterals = Literal["STEP", "XML", "GLTF", "VTKJS", "VRML"]
+ExportLiterals = Literal["STEP", "XML", "GLTF", "VTKJS", "VRML", "STL"]
 
 PATH_DELIM = "/"
 
@@ -453,7 +453,7 @@ def save(
 
         if exportType is None:
             t = path.split(".")[-1].upper()
-            if t in ("STEP", "XML", "VRML", "VTKJS", "GLTF"):
+            if t in ("STEP", "XML", "VRML", "VTKJS", "GLTF", "STL"):
                 exportType = cast(ExportLiterals, t)
             else:
                 raise ValueError("Unknown extension, specify export type explicitly")
@@ -468,6 +468,8 @@ def save(
             exportGLTF(self, path, True, tolerance, angularTolerance)
         elif exportType == "VTKJS":
             exportVTKJS(self, path)
+        elif exportType == "STL":
+            self.toCompound().exportStl(path, tolerance, angularTolerance)
         else:
             raise ValueError(f"Unknown format: {exportType}")
 

tests/test_assembly.py

@@ -247,6 +247,7 @@ def test_toJSON(simple_assy, nested_assy, empty_top_assy):
         ("stp", ("STEP",)),
         ("caf", ("XML",)),
         ("wrl", ("VRML",)),
+        ("stl", ("STL",)),
     ],
 )
 def test_save(extension, args, nested_assy, nested_assy_sphere):