code: def validate_readable(option, value):
      
      if value is None:
          return value
      # First make sure its a string py3.3 open(True, 'r') succeeds
      # Used in ssl cert checking due to poor ssl module error reporting
      value = validate_string(option, value)
      open(value, 'r').close()
      return value
docs: Validates that 'value' is file-like and readable.