diff --git a/pydu/file.py b/pydu/file.py
index 399d39c..f7a1c20 100644
--- a/pydu/file.py
+++ b/pydu/file.py
@@ -118,6 +118,9 @@ def copy(src, dst, ignore_errors=False, follow_symlinks=True):
 
 
 def touch(path):
+    """
+    open a file as write,and then close it.
+    """
     with open(path, 'w'):
         pass