import pyautogui

def locate_first_center(pictures_path: list):
    """
    Args
        pictures_path(list): list of paths
    Returns
        Coordinates if one of the pictures is found.
        None if none of the pictures are found.
    """
    if len(pictures_path) < 1:
        raise IndexError
    for picture in pictures_path:
        center = pyautogui.locateCenterOnScreen(picture)
        if center:
            return center   # Coordinates of the found picture
    return center   # None or coordinates of the morst right picture.


if __name__ == "__main__":
    # a.pngがなければb.pngをクリックする
    pos = locate_first_center(['a.png', 'b.png'])
    click(pos)