diff -r d00910b286cd -r f61f5af5ed31 mercurial/ui.py
--- a/mercurial/ui.py	Fri Sep 22 22:45:02 2017 +0900
+++ b/mercurial/ui.py	Sat Sep 30 07:52:48 2017 -0700
@@ -1488,7 +1488,7 @@
 
     def _progclear(self):
         """clear progress bar output if any. use it before any output"""
-        if '_progbar' not in vars(self): # nothing loaded yet
+        if not haveprogbar(): # nothing loaded yet
             return
         if self._progbar is not None and self._progbar.printed:
             self._progbar.clear()
@@ -1792,3 +1792,6 @@
         # this is how the extension used to work but feel free to rework it.
         _progresssingleton = progress.progbar(ui)
     return _progresssingleton
+
+def haveprogbar():
+    return _progresssingleton is not None