Check binary existence in gclient: 2nd try.

gclient_utils.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Generic utils."""
 
 import errno
 import logging
 import os
 import Queue
@@ skipping 331 matching lines @@
 
   Accepts all subprocess2.Popen() parameters plus:
     print_stdout: If True, the command's stdout is forwarded to stdout.
     filter_fn: A function taking a single string argument called with each line
                of the subprocess2's output. Each line has the trailing newline
                character trimmed.
     stdout: Can be any bufferable output.
 
   stderr is always redirected to stdout.
   """
-  assert print_stdout or filter_fn
+  assert (print_stdout is not None) or filter_fn
   stdout = stdout or sys.stdout
   filter_fn = filter_fn or (lambda x: None)
   kid = subprocess2.Popen(
       args, bufsize=0, stdout=subprocess2.PIPE, stderr=subprocess2.STDOUT,
       **kwargs)
 
   # Do a flush of stdout before we begin reading from the subprocess2's stdout
   stdout.flush()
 
   # Also, we need to forward stdout to prevent weird re-ordering of output.
@@ skipping 403 matching lines @@
     keyvals = dict([x.strip() for x in l.split(':', 1)] for l in lines if l)
   except ValueError:
     raise Error(
         'Failed to process settings, please fix. Content:\n\n%s' % content)
   def fix_url(key):
     if keyvals.get(key):
       keyvals[key] = UpgradeToHttps(keyvals[key])
   fix_url('CODE_REVIEW_SERVER')
   fix_url('VIEW_VC')
   return keyvals