class_docstr: Container for different methods to open (un-)compressed files.
  
  `_FileOpeners` contains a dictionary that holds one method for each
  supported file format. Attribute lookup is implemented in such a way
  that an instance of `_FileOpeners` itself can be indexed with the keys
  of that dictionary. Currently uncompressed files as well as files
  compressed with ``gzip``, ``bz2`` or ``xz`` compression are supported.
  
  Notes
  -----
  `_file_openers`, an instance of `_FileOpeners`, is made available for
  use in the `_datasource` module.
  
  Examples
  --------
  >>> import gzip
  >>> np.lib._datasource._file_openers.keys()
  [None, '.bz2', '.gz', '.xz', '.lzma']
  >>> np.lib._datasource._file_openers['.gz'] is gzip.open
  True
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docstr: Return the keys of currently supported file openers.
  
  Parameters
  ----------
  None
  
  Returns
  -------
  keys : list
      The keys are None for uncompressed files and the file extension
      strings (i.e. ``'.gz'``, ``'.xz'``) for supported compression
      methods.
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code: def keys(self):
      """
      Return the keys of currently supported file openers.
  
      Parameters
      ----------
      None
  
      Returns
      -------
      keys : list
          The keys are None for uncompressed files and the file extension
          strings (i.e. ``'.gz'``, ``'.xz'``) for supported compression
          methods.
  
      """
      self._load()
      return list(self._file_openers.keys())
  
imports: import os
  from .._utils import set_module
label_desc: Use image node_id 3 for calling the _FileOpeners obj's underlying member method code with example usage: obj.keys() and returns: list