title: Finding python source internals
body: <p>I like to read the source of src.<strong>contains</strong>().
  As src is a python built-in class and it is open source I think it is available for inspection. I can see the definition in builtins.pyi. For the source I downloaded : git clone <a href="https://github.com/python/cpython/" rel="nofollow noreferrer">https://github.com/python/cpython/</a> and searched for &quot;class src(&quot;, I found nothing. In cpython/Lib/string.py I see a reference to &quot;import _string&quot; but again I am unable to find _string.py. Can you explain to me the procedure I have to follow to find a class implementation? Thanks.</p>