Return '' for unsupported export type

Fixed the bug where unsupported export types throw
400 assertion error instead of 400 exporter error.
Remove the misleading comment which wrongly claims
that `get_exporter_name()` is always called with
a supported exporter. Update tests accordingly.

Author: cslzchen

mfr/core/utils.py

@@ -112,12 +112,11 @@ def get_renderer_name(name: str) -> str:
     ep_iterator = pkg_resources.iter_entry_points(group='mfr.renderers', name=name.lower())
     ep_list = list(ep_iterator)
 
-    # Empty list indicates unsupported file type.
-    # Return a blank string and let `make_renderer()` handle it.
+    # Empty list indicates unsupported file type.  Return '' and let `make_renderer()` handle it.
     if len(ep_list) == 0:
         return ''
 
-    # If file type is supported, there must be only one element in the list.
+    # If the file type is supported, there must be only one element in the list.
     assert len(ep_list) == 1
     return ep_list[0].attrs[0]
 
@@ -135,7 +134,11 @@ def get_exporter_name(name: str) -> str:
     ep_iterator = pkg_resources.iter_entry_points(group='mfr.exporters', name=name.lower())
     ep_list = list(ep_iterator)
 
-    # `make_renderer()` is called before `make_exporter()` to ensure the file type is supported
+    # Empty list indicates unsupported export type.  Return '' and let `make_exporter()` handle it.
+    if len(ep_list) == 0:
+        return ''
+
+    # If the export type is supported, there must be only one element in the list.
     assert len(ep_list) == 1
     return ep_list[0].attrs[0]
 

tests/core/test_utils.py

@@ -20,7 +20,7 @@ def test_get_renderer_name(self):
             assert mfr_utils.get_renderer_name(ep.name) == expected
 
     def test_get_renderer_name_no_entry_point(self):
-            assert mfr_utils.get_renderer_name('jpg') == ''
+        assert mfr_utils.get_renderer_name('jpg') == ''  # extensions must begin with a period
 
 
 class TestGetExporterName:
@@ -36,5 +36,4 @@ def test_get_exporter_name(self):
             assert mfr_utils.get_exporter_name(ep.name) == expected
 
     def test_get_exporter_name_no_entry_point(self):
-        with pytest.raises(AssertionError):
-            mfr_utils.get_exporter_name('jpg')
+        assert mfr_utils.get_exporter_name('jpg') == ''  # extensions must begin with a period