diff --git a/test/shaping/hb-diff b/test/shaping/hb-diff
index 81ef7d90c..46ad19c31 100755
--- a/test/shaping/hb-diff
+++ b/test/shaping/hb-diff
@@ -10,7 +10,7 @@ if "--color" in sys.argv or os.isatty (sys.stdout.fileno ()):
 	green_color = '\033[42;37;1m'
 	end_color = '\033[m'
 
-def fancy_diff (l1, l2):
+def fancy_diff_lines (l1, l2):
 
 	ss = [re.sub ('([A-Za-z0-9_]*)([^A-Za-z0-9_]?)', r'\1\n\2\n', l).splitlines (True) for l in (l1, l2)]
 	oo = ["",""]
@@ -45,26 +45,31 @@ def fancy_diff (l1, l2):
 		return [' ', oo[0], '\n']
 	return ['-', oo[0], '\n', '+', oo[1], '\n']
 
+def fancy_diff_files (f1, f2):
+	for (l1,l2) in zip (f1, f2):
+		if l1 == l2:
+			sys.stdout.writelines ([" ", l1])
+			continue
+
+		sys.stdout.writelines (fancy_diff_lines (l1, l2))
+	# Print out residues
+	for l in f1:
+		sys.stdout.writelines (["-", red_color, l1, end_color])
+	for l in f1:
+		sys.stdout.writelines (["-", green_color, l1, end_color])
+
 
 def open_file (f):
 	if f == '-':
 		return sys.stdin
 	return file (f)
 
-if len (sys.argv) != 3:
-	print "Usage: %s [--color] FILE1 FILE2" % sys.argv[0]
-	sys.exit (1)
+if __name__ == '__main__':
 
-f1, f2 = (open_file (f) for f in sys.argv[1:3])
+	if len (sys.argv) != 3:
+		print "Usage: %s [--color] FILE1 FILE2" % sys.argv[0]
+		sys.exit (1)
 
-for (l1,l2) in zip (f1, f2):
-	if l1 == l2:
-		sys.stdout.writelines ([" ", l1])
-		continue
+	f1, f2 = (open_file (f) for f in sys.argv[1:3])
 
-	sys.stdout.writelines (fancy_diff (l1, l2))
-# Print out residues
-for l in f1:
-	sys.stdout.writelines (["-", red_color, l1, end_color])
-for l in f1:
-	sys.stdout.writelines (["-", green_color, l1, end_color])
+	fancy_diff_files (f1, f2)