pytest-3.6.4/src/_pytest/pytester.py

Python source file Reference

A .py file contains Python source code in plain text. The Python interpreter reads the file and runs the code directly, without a separate compile step. Developers write scripts, modules, and full programs in this format. A text editor can open and change the file. Python is one of the most used programming languages, thus .py files are common on many systems.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.