Fix warnings in the diff example (google#135)

BoboTiG · dbieber · commit 54f91a21c06a · 2018-08-28T16:21:50.000-07:00
Fix ResourceWarning (unclosed file) and DeprecationWarning ('U' mode is deprecated)
diff --git a/examples/diff/diff.py b/examples/diff/diff.py
@@ -74,8 +74,10 @@ def __init__(self, fromfile, tofile):
 
     self.fromdate = time.ctime(os.stat(fromfile).st_mtime)
     self.todate = time.ctime(os.stat(tofile).st_mtime)
-    self.fromlines = open(fromfile, 'U').readlines()
-    self.tolines = open(tofile, 'U').readlines()
+    with open(fromfile) as f:
+      self.fromlines = f.readlines()
+    with open(tofile) as f:
+      self.tolines = f.readlines()
 
   def unified_diff(self, lines=3):
     return difflib.unified_diff(
