import string
import random


def id_gen(id_length):
    # type: (int) -> str
    """Generate a unique alpha-numeric ID of a specified length.
    
    :param id_length: Length of unique string.
    :return: Unique string.
    """

    selection = string.ascii_letters + string.digits
    return ''.join(random.choice(selection) for _ in range(id_length))