--- a/testing/mozharness/mozharness/base/vcs/mercurial.py
+++ b/testing/mozharness/mozharness/base/vcs/mercurial.py
@@ -383,17 +383,18 @@ class MercurialVCS(ScriptMixin, LogMixin
 
         if rev:
             args.extend(['--revision', rev])
         if branch:
             args.extend(['--branch', branch])
 
         parser = RepositoryUpdateRevisionParser(config=self.config,
                                                 log_obj=self.log_obj)
-        if self.run_command(args, output_parser=parser):
+        if self.run_command(args, output_parser=parser,
+                            partial_env={'PYTHONUNBUFFERED': '1'}):
             raise VCSException('repo checkout failed!')
 
         if not parser.revision:
             raise VCSException('could not identify revision updated to')
 
         return parser.revision
 
     def apply_and_push(self, localrepo, remote, changer, max_attempts=10,