def _check_unpack_options(extensions, function, extra_args):
<EOL>existing_extensions = {}<EOL>for name, info in _UNPACK_FORMATS.items():<EOL><INDENT>for ext in info[<NUM_LIT:0>]:<EOL><INDENT>existing_extensions[ext] = name<EOL><DEDENT><DEDENT>for extension in extensions:<EOL><INDENT>if extension in existing_extensions:<EOL><INDENT>msg = '<STR_LIT>'<EOL>raise RegistryError(msg % (extension,<EOL>existing_extensions[extension]))<EOL><DEDENT><DEDENT>if not isinstance(function, collections.Callable):<EOL><INDENT>raise TypeError('<STR_LIT>')<EOL><DEDENT>
Checks what gets registered as an unpacker.