remove instance UFOs before calling mutatormath

Fixes #372

Lib/fontmake/font_project.py

@@ -20,6 +20,7 @@
 import logging
 import math
 import os
+import shutil
 import tempfile
 try:
  from plistlib import load as readPlist # PY3
@@ -411,6 +412,14 @@ def run_from_designspace(
  ufos.extend(reader.getSourcePaths())
  if interpolate:
  logger.info('Interpolating master UFOs from designspace')
+ _, ilocs = self._designspace_locations(designspace_path)
+ for ipath in ilocs.keys():
+ # mutatorMath does not remove the existing instance UFOs
+ # so we do it ourselves, or else strange things happen...
+ # https://github.com/googlei18n/fontmake/issues/372
+ if ipath.endswith(".ufo") and os.path.isdir(ipath):
+ logger.debug("Removing %s" % ipath)
+ shutil.rmtree(ipath)
  results = build_designspace(
  designspace_path, outputUFOFormatVersion=3,
  roundGeometry=not no_round)