text: # Copyright 2015 gRPC authors.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  """Helper to watch a (set) of directories for modifications."""
  
  import os
  import time
  
  
  class DirWatcher(object):
      """Helper to watch a (set) of directories for modifications."""
  
      def __init__(self, paths):
          if isinstance(paths, basestring):
              paths = [paths]
          self._done = False
          self.paths = list(paths)
          self.lastrun = time.time()
          self._cache = self._calculate()
  
      def _calculate(self):
          """Walk over all subscribed paths, check most recent mtime."""
          most_recent_change = None
          for path in self.paths:
              if not os.path.exists(path):
                  continue
              if not os.path.isdir(path):
                  continue
              for root, _, files in os.walk(path):
                  for f in files:
                      if f and f[0] == '.': continue
                      try:
                          st = os.stat(os.path.join(root, f))
                      except OSError as e:
                          if e.errno == os.errno.ENOENT:
                              continue
                          raise
                      if most_recent_change is None:
                          most_recent_change = st.st_mtime
                      else:
                          most_recent_change = max(most_recent_change,
                                                   st.st_mtime)
          return most_recent_change
  
      def most_recent_change(self):
          if time.time() - self.lastrun > 1:
              self._cache = self._calculate()
              self.lastrun = time.time()
          return self._cache
  