def DEFAULT_NULLVALUEFORMAT(format):
    """
    Returns a null value for each of the various kinds of numpy formats.

    Default null value function used in :func:`tabular.spreadsheet.join`.

    **Parameters**

            **format** :  string

                    Numpy format descriptor, e.g. ``'<i4'``, ``'|S5'``.

    **Returns**

            **null** :  element in `[0, 0.0, '']`

                    Null value corresponding to the given format:

                    *   if ``format.startswith(('<i', '|b'))``, e.g. `format`
                        corresponds to an integer or Boolean, return 0

                    *   else if `format.startswith('<f')`, e.g. `format`
                        corresponds to a float, return 0.0

                    *   else, e.g. `format` corresponds to a string, return ''

    """
    return 0 if format.startswith(('<i','|b')) \
           else 0.0 if format.startswith('<f') \
           else ''