merging in layer import fix

Author: isaisabel

CHANGELOG.md

@@ -3,6 +3,7 @@
 - [diff_stix.py](scripts/diff_stix.py) now supports an `--unchanged` argument which adds a section listing objects which did _not_ change between releases.
 ## Fixes
 - [diff_stix.py](scripts/diff_stix.py) should no longer crash when exporting layers but with only one domain specified.
+- Fixed a bug in the layer library where layers could be imported improperly in specific contexts.
 
 # v1.8.1 - 3 March 2021
 ## Fixes

layers/exporters/svg_objects.py

@@ -260,7 +260,7 @@ def build(height, width, label, config, variant='text', t1text=None, t2text=None
             upper = G(tx=0, ty=2.1)
             internal.append(upper)
             if t1text is not None:
-                bu = t2text is not None and t2text is not ""
+                bu = t2text != None and t2text != ""
                 theight = (height-5)
                 if bu:
                     theight = theight / 2

layers/exporters/to_svg.py

@@ -7,6 +7,8 @@
     from ..core import Layer
     from ..exporters.svg_templates import SvgTemplates
 
+from layers.core import Layer as topLayer # alternative import for typechecking
+
 class NoLayer(Exception):
     pass
 
@@ -405,7 +407,7 @@ def to_svg(self, layer, filepath='example.svg'):
             :return: (meta) svg file at the targeted output location
         """
         if layer is not None:
-            if not isinstance(layer, Layer):
+            if not isinstance(layer, Layer) and not isinstance(layer, topLayer):
                 raise TypeError
 
         if layer is None: