input: Write a function to read the contents from a given URL and return the text if the request is successful; otherwise, return None.
  
  def read_url(url):
    r = requests.get(url)
    if r.status_code == 200:
      return r.text
    else:
      return None
  
  assert read_url('http://example.com') is not None
  assert read_url('http://invalidurl123456.com') is None
output: def read_url(url):
    r = requests.get(url)
    if r.status_code == 200:
      return r.text
    else:
      return None