Wrong output in python3 via terminal on mac

[snippsat]

However, each time I run the code it keeps asking me to enter file name while there is no input line in my file.

It should not do that,try running from Terminal command line if you run from a IDE/editor.
Even if fix import the code will never work because tags = soup('a'),there are no links(which a search for) on that page.
To show a example that work.

import urllib.request
from bs4 import BeautifulSoup

url = 'http://py4e-data.dr-chuck.net/comments_42.html'
html = urllib.request.urlopen(url)
soup = BeautifulSoup(html, "html.parser")

td_tag = soup.find_all('span', class_="comments")
total = sum(int(td.text) for td in td_tag)
print('total is {}'.format(total))

Output:
total is 2553

Should also drop urllib and use Requests.