def validate_number(value_if_allowed):
    """
    Validation method used by Tkinter. Accepts empty and float-coercable
    strings.

    Params:
        value_if_allowed
            Value to validate.

    Returns:
        True if value_if_allowed is empty, or can be interpreted as a float.
    """
    if value_if_allowed == '':
        return True
    try:
        float(value_if_allowed)
        return True
    except ValueError:
        return False