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It's just about backward compatibility. If you don't run the exact version of python that's running in production, how do you know that you're not using some method that does not exist in production yet (because you run older version there)?

Also, libraries with binary component often have to be compiled against specific version of python.

Sometimes! A very simple example is code that uses "async" as a variable name. It became a keyword in 3.5, which was an enormous pain in the ass.

Because underscore-functions aren't truly private, I have once seen an upgrade from 2.7.8 to 2.7.13 fail. A commonly-used package was importing one from a core python module.