input: Write a function to read the contents from a given URL and return the text content if the request is successful, otherwise return None if the request fails.
  
  import requests
  
  def read_url(url):
    r = requests.get(url)
    if r.status_code == 200:
      return r.text
    else:
      return None
  
  assert read_url("https://www.example.com") is not None
  assert read_url("https://www.nonexistentwebsite.com") is None
output: 
    r = requests.get(url)
    if r.status_code == 200:
      return r.text
    else:
      return None
  