improve error messages for artist creation

Author: gonzalocasas

CHANGELOG.md

@@ -51,6 +51,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Fixed `compas_blender.artists.VolMeshArtist.draw` and `compas_blender.artists.VolMeshArtist.draw_cells`.
 * Fixed `compas_ghpython.artists.VolMeshArtist.draw` and `compas_ghpython.artists.VolMeshArtist.draw_cells`.
 * Fixed `compas_rhino.artists.VolMeshArtist.draw` and `compas_rhino.artists.VolMeshArtist.draw_cells`.
+* Improved error messages when artist instance cannot be created.
 
 ### Removed
 

src/compas/artists/artist.py

@@ -45,6 +45,13 @@ def _get_artist_cls(data, **kwargs):
     else:
         Artist.CONTEXT = identify_context()
 
+    if Artist.CONTEXT is None:
+        error_message = 'No context defined.'
+        error_message += '\n\nThis usually means that the script that you are running requires'
+        error_message += '\na CAD environment but it is being ran as a standalone script'
+        error_message += '\n(ie. from the command line or code editor).'
+        raise Exception(error_message)
+
     dtype = type(data)
     cls = None
 
@@ -89,6 +96,10 @@ def __new__(cls, *args, **kwargs):
         if not Artist.__ARTISTS_REGISTERED:
             register_artists()
             Artist.__ARTISTS_REGISTERED = True
+
+        if not args or len(args) < 1 or args[0] is None:
+            raise ValueError('Cannot create an artist for `None`. Please ensure you pass a instance of a supported class.')
+
         cls = _get_artist_cls(args[0], **kwargs)
         PluginValidator.ensure_implementations(cls)
         return super(Artist, cls).__new__(cls)