[varLib.avar] Don't require a full .designspace with sources

Lib/fontTools/varLib/__init__.py

@@ -869,7 +869,7 @@ def _add_COLR(font, model, master_fonts, axisTags, colr_layer_reuse=True):
         colr.VarIndexMap = builder.buildDeltaSetIndexMap(varIdxes)
 
 
-def load_designspace(designspace, log_enabled=True):
+def load_designspace(designspace, log_enabled=True, *, require_sources=True):
     # TODO: remove this and always assume 'designspace' is a DesignSpaceDocument,
     # never a file path, as that's already handled by caller
     if hasattr(designspace, "sources"):  # Assume a DesignspaceDocument
@@ -878,7 +878,7 @@ def load_designspace(designspace, log_enabled=True):
         ds = DesignSpaceDocument.fromfile(designspace)
 
     masters = ds.sources
-    if not masters:
+    if require_sources and not masters:
         raise VarLibValidationError("Designspace must have at least one source.")
     instances = ds.instances
 
@@ -978,7 +978,7 @@ def load_designspace(designspace, log_enabled=True):
                     "More than one base master found in Designspace."
                 )
             base_idx = i
-    if base_idx is None:
+    if require_sources and base_idx is None:
         raise VarLibValidationError(
             "Base master not found; no master at default location?"
         )

Lib/fontTools/varLib/avar.py

@@ -150,7 +150,7 @@ def main(args=None):
 
     axisTags = [a.axisTag for a in font["fvar"].axes]
 
-    ds = load_designspace(options.designspace)
+    ds = load_designspace(options.designspace, require_sources=False)
 
     if "avar" in font:
         log.warning("avar table already present, overwriting.")