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

from .errors import InvalidValue
from .errors import InvalidCellValue

def valid_value(val):
    """Checks whether `val` is a valid value for averages"""
    if re.search(r"^[0-9]+\.[0-9]{3}$", val):
        return float(val)
    raise InvalidCellValue(
        f"Invalid value '{val}'. "
        "Expected string representing a number with exactly three decimal "
        "places.")

def invalid_value(line_number: int, column_number: int, val: str) -> Union[
        InvalidValue, None]:
    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."))