code: def check_for_credential_file(self):
          
          if 'AWS_CREDENTIAL_FILE' in os.environ:
              path = os.environ['AWS_CREDENTIAL_FILE']
              path = os.path.expanduser(path)
              path = os.path.expandvars(path)
              if os.path.isfile(path):
                  fp = open(path)
                  lines = fp.readlines()
                  fp.close()
                  for line in lines:
                      if line[0] != '#':
                          if '=' in line:
                              name, value = line.split('=', 1)
                              if name.strip() == 'AWSAccessKeyId':
                                  if 'aws_access_key_id' not in self.args:
                                      value = value.strip()
                                      self.args['aws_access_key_id'] = value
                              elif name.strip() == 'AWSSecretKey':
                                  if 'aws_secret_access_key' not in self.args:
                                      value = value.strip()
                                      self.args['aws_secret_access_key'] = value
              else:
                  print 'Warning: unable to read AWS_CREDENTIAL_FILE'
docs: Checks for the existance of an AWS credential file.
          If the environment variable AWS_CREDENTIAL_FILE is
          set and points to a file, that file will be read and
          will be searched credentials.
          Note that if credentials have been explicitelypassed
          into the class constructor, those values always take
          precedence.