def read_catigories():
    """Returns catigories, None if there are none."""
    # LXML stuff
    return page.get_catigories()

def read_products():
    """Returns products, None if there are none."""
    # LXML stuff
    return page.get_products()

def read_all_catigories():
    cat = read_catigories()
    if cat is None:
        yield read_products()
    else:
        yield from read_all_catigories()

def read_all_categories(page):
    cat = read_categories(page)
    if not cat:
        yield read_products(page)
    else:
        for link in cat:
            yield from read_all_categories(link)