Make pretty error messages for LexerError

Lib/fontTools/feaLib/lexer.py

@@ -9,6 +9,14 @@ class LexerError(Exception):
         Exception.__init__(self, message)
         self.location = location
 
+    def __str__(self):
+        message = Exception.__str__(self)
+        if self.location:
+            path, line, column = self.location
+            return "%s:%d:%d: %s" % (path, line, column, message)
+        else:
+            return message
+
 
 class Lexer(object):
     NUMBER = "NUMBER"

Lib/fontTools/feaLib/lexer_test.py

@@ -9,6 +9,16 @@ def lex(s):
     return [(typ, tok) for (typ, tok, _) in Lexer(s, "test.fea")]
 
 
+class LexerErrorTest(unittest.TestCase):
+    def test_str(self):
+        err = LexerError("Squeak!", ("foo.fea", 23, 42))
+        self.assertEqual(str(err), "foo.fea:23:42: Squeak!")
+
+    def test_str_nolocation(self):
+        err = LexerError("Squeak!", None)
+        self.assertEqual(str(err), "Squeak!")
+
+
 class LexerTest(unittest.TestCase):
     def test_empty(self):
         self.assertEqual(lex(""), [])