input: def read_url(url: str) -> str:
      """
      Read the contents from a given URL and return the text if the request is successful.
      If the request fails, return None.
  
      >>> read_url("http://example.com")
      'Example Domain\\nThis domain is for use in illustrative examples in documents. [...]'
      >>> read_url("http://nonexistenturl")
      None
      """
      import requests
  
      r = requests.get(url)
      if r.status_code == 200:
          return r.text
      else:
          return None
output:     import requests
  
      r = requests.get(url)
      if r.status_code == 200:
          return r.text
      else:
          return None