userNameToFileName expects userName to be unicode strings, let it raise

if not

Lib/ufoLib/__init__.py

@@ -3,6 +3,7 @@ import shutil
 from io import StringIO, BytesIO, open
 import codecs
 from copy import deepcopy
+from fontTools.misc.py23 import basestring, unicode
 from ufoLib.glifLib import GlyphSet
 from ufoLib.validators import *
 from ufoLib.filenames import userNameToFileName
@@ -38,11 +39,6 @@ fontinfo.plist values between the possible format versions.
 	convertFontInfoValueForAttributeFromVersion3ToVersion2
 """
 
-try:
-	basestring
-except NameError:
-	basestring = str
-
 __all__ = [
 	"makeUFOPath"
 	"UFOLibError",
@@ -1066,9 +1062,9 @@ class UFOWriter(object):
 				# not caching this could be slightly expensive,
 				# but caching it will be cumbersome
 				existing = [d.lower() for d in list(self.layerContents.values())]
-				if not isinstance(layerName, basestring):
+				if not isinstance(layerName, unicode):
 					try:
-						layerName = str(layerName)
+						layerName = unicode(layerName)
 					except UnicodeDecodeError:
 						raise UFOLibError("The specified layer name is not a Unicode string.")
 				directory = userNameToFileName(layerName, existing=existing, prefix="glyphs.")

Lib/ufoLib/filenames.py

@@ -2,7 +2,7 @@
 User name to file name conversion.
 This was taken form the UFO 3 spec.
 """
-from fontTools.misc.py23 import unicode
+from fontTools.misc.py23 import basestring, unicode
 
 
 illegalCharacters = "\" * + / : < > ? [ \ ] | \0".split(" ")
@@ -68,7 +68,8 @@ def userNameToFileName(userName, existing=[], prefix="", suffix=""):
 	u'alt._con'
 	"""
 	# the incoming name must be a unicode string
-	assert isinstance(userName, unicode), "The value for userName must be a unicode string."
+	if not isinstance(userName, unicode):
+		raise ValueError("The value for userName must be a unicode string.")
 	# establish the prefix and suffix lengths
 	prefixLength = len(prefix)
 	suffixLength = len(suffix)

Lib/ufoLib/glifLib.py

@@ -15,7 +15,7 @@ from __future__ import unicode_literals
 import os
 from io import BytesIO, open
 from warnings import warn
-from fontTools.misc.py23 import tobytes
+from fontTools.misc.py23 import tobytes, unicode
 from ufoLib.plistlib import PlistWriter, readPlist, writePlist
 from ufoLib.plistFromETree import readPlistFromTree
 from ufoLib.pointPen import AbstractPointPen, PointToSegmentPen
@@ -457,9 +457,9 @@ def glyphNameToFileName(glyphName, glyphSet):
 		existing = [name.lower() for name in list(glyphSet.contents.values())]
 	else:
 		existing = []
-	if not isinstance(glyphName, basestring):
+	if not isinstance(glyphName, unicode):
 		try:
-			new = str(glyphName)
+			new = unicode(glyphName)
 			glyphName = new
 		except UnicodeDecodeError:
 			pass