code: def check_file_exists(self, filename, resolve='exception', force=None):
          
          def _warn(x):
              msg = "File {0!r} already exists.".format(x)
              logger.warn(msg)
              warnings.warn(msg)
              return True
          def _raise(x):
              msg = "File {0!r} already exists.".format(x)
              logger.error(msg)
              raise IOError(errno.EEXIST, x, msg)
          solutions = {'ignore': lambda x: False,      # file exists, but we pretend that it doesn't
                       'indicate': lambda x: True,     # yes, file exists
                       'warn': _warn,
                       'warning': _warn,
                       'exception': _raise,
                       'raise': _raise,
                       }
  
          if force is True:
              resolve = 'ignore'
          elif force is False:
              resolve = 'exception'
  
          if not os.path.isfile(filename):
              return False
          else:
              return solutions[resolve](filename)
docs: If a file exists then continue with the action specified in ``resolve``.
  
          ``resolve`` must be one of
  
          "ignore"
                always return ``False``
          "indicate"
                return ``True`` if it exists
          "warn"
                indicate and issue a :exc:`UserWarning`
          "exception"
                raise :exc:`IOError` if it exists
  
          Alternatively, set *force* for the following behaviour (which
          ignores *resolve*):
  
          ``True``
                same as *resolve* = "ignore" (will allow overwriting of files)
          ``False``
                same as *resolve* = "exception" (will prevent overwriting of files)
          ``None``
                ignored, do whatever *resolve* says