This is not a question specific to defaultdict but in general philosophy or logic behind placing code in test files.
I would like to know reason behind placing the defaultdict class in cpython/Lib/test/test_descrtut.py. Github link (https://github.com/python/cpython/blob/master/Lib/test/test_descrtut.py)
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.
As this file is for test code the question is, why the defaultdict class in present in this file.
I tried going to documentation but could not easily figure out the reason. https://devguide.python.org/exploring/
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