title: Python __attrs__ explained
body: <p>I am going through the code of Python <code>requests</code> library from Kenneth Reitz (which is awesome!). And I have encountered a Class variable named <code>__attrs__</code> (see below). Tried to find out something about it via Google and SymbolHound, but no luck.</p>
  
  <p>Is this a standard Python thing? Where can I find more infos? Can someone enlighten me?</p>
  
  <p>From: <a href="https://github.com/kennethreitz/requests/blob/master/requests/sessions.py" rel="noreferrer">https://github.com/kennethreitz/requests/blob/master/requests/sessions.py</a></p>
  
  <pre><code>class Session(SessionRedirectMixin):
      ...
  
      __attrs__ = [
          'headers', 'cookies', 'auth', 'proxies', 'hooks', 'params', 'verify',
          'cert', 'prefetch', 'adapters', 'stream', 'trust_env',
          'max_redirects',
      ]
  
      def __init__(self):
  
          #: A case-insensitive dictionary of headers to be sent on each
          #: :class:`Request &lt;Request&gt;` sent from this
          #: :class:`Session &lt;Session&gt;`.
          self.headers = default_headers()
  </code></pre>