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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code: def __getitem__(self, key):
      self._load()
      return self._file_openers[key]
  
imports: import os
  import io
  from .._utils import set_module
label_desc: Use image node_id 4 for calling the _FileOpeners obj's underlying member method code with example usage: obj.__getitem__(key) and returns: self