old_contents: from __future__ import absolute_import, division, print_function
  
  __all__ = (
      'KeyInvalidError',
      'KeyFileNotFoundError',
      'KeyExpirationError',
      'BackendLoadError',
  )
  
  
  class KeyInvalidError(Exception):
      """Raised internally when a key cannot be loaded."""
      pass
  
  
  class KeyFileNotFoundError(KeyInvalidError):
      """Raised when file for key doesn't exist."""
      pass
  
  
  class KeyExpirationError(KeyInvalidError):
      """Raised when key has expired."""
      pass
  
  
  class BackendLoadError(Exception):
      """Raised when Backend.from_file cannot load an object."""
      pass
  
new_contents: from __future__ import absolute_import, division, print_function
  
  __all__ = (
      'KeyInvalidError',
      'KeyFileNotFoundError',
      'KeyExpirationError',
      'BackendLoadError',
  )
  
  
  class KeyInvalidError(Exception):
      """Raised internally when a key cannot be loaded."""
  
  
  class KeyFileNotFoundError(KeyInvalidError):
      """Raised when file for key doesn't exist."""
  
  
  class KeyExpirationError(KeyInvalidError):
      """Raised when key has expired."""
  
  
  class BackendLoadError(Exception):
      """Raised when :py:meth:`bucketcache.backends.Backend.from_file` cannot
      load an object.
      """
  
subject: Fix documentation error