A wrapper over str(), but converts bool values to lower case strings.
    If None is given, just returns None, instead of converting it to string "None".

def to_str(value):
    """
    A wrapper over str(), but converts bool values to lower case strings.
    If None is given, just returns None, instead of converting it to string "None".
    """
    if isinstance(value, bool):
        return str(value).lower()
    elif value is None:
        return value
    else:
        return str(value)