def check_async(async_, kwargs, default):
if async_ is not None:<EOL><INDENT>return async_<EOL><DEDENT>elif '<STR_LIT>' in kwargs:<EOL><INDENT>warnings.warn(<EOL>'<STR_LIT>',<EOL>DeprecationWarning,<EOL>)<EOL>return kwargs.pop('<STR_LIT>')<EOL><DEDENT>else:<EOL><INDENT>return default<EOL><DEDENT>
Return a value of 'async' in kwargs or default when async_ is None.

    This helper function exists for backward compatibility (See #274).
    It shows a warning message when 'async' in kwargs is used to note users.