title: How to Python count list length
body: <p>I want to know how len() works.</p>
  
  <p>Does it count from beginning to end of a list every time I call len(), or, since list is also an class, does len() just return a variable in the list object which record the list's length?</p>
  
  <p>Also, I hope someone can tell me where I can find the source code of those built-in functions like 'len()', 'map()', etc.</p>