"""Contain logic for checking average files"""
import re

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}'.\n"
        "Expected string representing a number with exactly three decimal "
        "places.")