只有一个参数时Python抛出多个参数错误

我在另一个类中调用一个方法,得到以下错误。这是声明定义方法的类:

from web import Web
import ast


class WebCrawler:
    web = Web()

    def crawl(self):
        root = self.root()
        URL = ast.literal_eval(self.get(root))
        return URL

这个类称之为:

from web import Web
from crawler import WebCrawler

web = Web()
crawler = WebCrawler()
urls = crawler.crawl(web)

print(urls)

Traceback (most recent call last):
  File "/home/onur/Desktop/web-crawler/test-run.py", line 6, in <module>
    urls = crawler.crawl(web)
TypeError: crawl() takes 1 positional argument but 2 were given