import re


def clean_python_variable_name(s):
    """ Convert a string to a valid python variable name.

    Examples::

    >>> clean_python_variable_name("    ")
    '____'
    >>> clean_python_variable_name("my 2@'(")
    'my_2___'
    >>> clean_python_variable_name("my superS@---variable")
    'my_superS____variable'
    """
    return re.sub('\W|^(?=\d)', '_', s)