input: Write a python function to extract comments from a given Reddit post URL and return them as a list of strings.
  assert len(extract_comments("https://www.reddit.com/r/ProgrammerHumor/comments/hfpmoo/people_ask_me_why_i_learn_so_many_programming/")) > 0
  assert isinstance(extract_comments("https://www.reddit.com/r/ProgrammerHumor/comments/hfpmoo/people_ask_me_why_i_learn_so_many_programming/"), list)
  assert all(isinstance(comment, str) for comment in extract_comments("https://www.reddit.com/r/ProgrammerHumor/comments/hfpmoo/people_ask_me_why_i_learn_so_many_programming/"))
output: def extract_comments(reddit_post_url):
      import requests
      import json
  
      headers = {'User-Agent': 'Mozilla/5.0'}
      response = requests.get(reddit_post_url + ".json", headers=headers)
      if response.status_code != 200:
          return []
  
      data = response.json()
  
      comments = []
      if len(data) > 1 and 'data' in data[1] and 'children' in data[1]['data']:
          for comment in data[1]['data']['children']:
              if 'data' in comment and 'body' in comment['data']:
                  comments.append(comment['data']['body'])
  
      return comments