"""Contain logic for checking average files"""
import re
from typing import Union

from .errors import InvalidValue

def invalid_value(line_number: int, column_number: int, val: str) -> Union[
        InvalidValue, None]:
    """Return an `InvalidValue` object if `val` is not a valid "averages"
    value."""
    if re.search(r"^[0-9]+\.[0-9]{3}$", val):
        return None
    return InvalidValue(
        line_number, column_number, val, (
            f"Invalid value '{val}'. "
            "Expected string representing a number with exactly three decimal "
            "places."))