title: question about python code folder (lib/test) structure
body: <p>This is not a question specific to defaultdict but in general philosophy or logic behind placing  code in test files.</p>
  <p>I would like to know reason behind placing the defaultdict class in cpython/Lib/<strong>test</strong>/test_descrtut.py. Github link (<a href="https://github.com/python/cpython/blob/master/Lib/test/test_descrtut.py" rel="nofollow noreferrer">https://github.com/python/cpython/blob/master/Lib/test/test_descrtut.py</a>)</p>
  <p>The third subfolder is named test and also the file name has test in it, which indicates that its for test code. Also it has test code.</p>
  <p>As this file is for test code the question is, why the defaultdict class in present in this file.</p>
  <p>I tried going to documentation but could not easily figure out the reason. <a href="https://devguide.python.org/exploring/" rel="nofollow noreferrer">https://devguide.python.org/exploring/</a></p>