def default_format(fmt=None):
    """Get or set the default ouptut format.

    Include a fmt if and where you need to specify the output
    precision.  Defaults to %.*f, where the * stands for the
    precision.  Do nothing if fmt is None.

    Returns: default format.

    >>> print mg(2, 'm2').sqrt()
    1.4142 m
    >>> default_format("%.2f")
    '%.2f'
    >>> print mg(2, 'm2').sqrt()
    1.41 m
    >>> reset_default_format()
    """
    global _prn_format
    if fmt is not None:
        _prn_format = fmt
    return _prn_format
