diff --git a/.gitignore b/.gitignore
index 679e5efb..a42aa2ac 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,4 @@
 test*.py
 Krakatau/plugins/*
 tests/.cache
+.idea/
diff --git a/assemble.py b/assemble.py
index db4f26ab..1d2567f4 100755
--- a/assemble.py
+++ b/assemble.py
@@ -2,6 +2,7 @@
 from __future__ import print_function
 
 import os.path, time
+import sys
 
 import Krakatau
 from Krakatau.assembler import parse
@@ -13,7 +14,11 @@ def assembleSource(source, basename, fatal=False):
 
 def assembleClass(filename):
     basename = os.path.basename(filename)
-    with open(filename, 'rU') as f:
+    if sys.version_info < (3,):
+        mode = 'rU'
+    else:  # python 3 deprecates mode "U" in favor of "newline" option
+        mode = 'r'
+    with open(filename, mode) as f:
         source = f.read()
     return assembleSource(source, basename)