def strToFloatOrNone(str):
    """Returns float representation of str argument or None if empty string.

    :param str: String to ne converted into float number
    :return: Float representation of str or None.


    >>> print strToFloatOrNone("36")
    36.0

    >>> print strToFloatOrNone("")
    None

    """
    if str:
        return float(str)
    else:
        return None