__init__.py from matplotlib

def _string_to_bool(s):
    """Parses the string argument as a boolean"""
    if not isinstance(s, str):
        return bool(s)
    warn_deprecated("2.2", "Passing one of 'on', 'true', 'off', 'false' as a "
                    "boolean is deprecated; use an actual boolean "
                    "(True/False) instead.")
    if s.lower() in ['on', 'true']:
        return True
    if s.lower() in ['off', 'false']:
        return False
    raise ValueError('String "%s" must be one of: '
                     '"on", "off", "true", or "false"' % s)