But imagine what would happen if they <i>had</i> created a compatibility layer.  Yes, it would be very hacky, and require a lot of seemingly gratuitous work - tons of standard library functions would have to check the compatibility version of the caller and change behavior accordingly.  And the goal of cleaning up the codebase by removing things like old-style classes wouldn&#x27;t be achievable.  But what if that work had been done, and the mess accepted?<p>Then Python 3 code could use every unmodified Python 2 library (albeit potentially requiring the former to do some messy conversions between str and bytes when making calls), including the long tail of legacy libraries that will never be upgraded.  Meanwhile, once the issues in the first Python 3 release or two were straightened out, there would be very little reason to use the Python 2 <i>interpreter</i>, since the Python 3 one would be a drop-in replacement; so migration to the latter would be relatively swift, and it wouldn&#x27;t be too much of a burden to ask someone to upgrade as a prerequisite to using your code.  So there would be very little reason to write new Python code in the Python 2 dialect (unless you really really hate making print a function :), and existing code could be migrated one file at a time.  Including popular libraries that wanted to maintain backwards compatibility with Python 2 programs - they would have to add compatibility checks similar to the standard library, but could otherwise go full Python 3, rather than having to use the awkward intersection of language semantics and compatibility libraries currently required.<p>With the transition so much smoother overall, native Python 3 code would probably take over relatively quickly.  (Maybe it would have even been feasible to remove the Python 2 compatibility layer eventually, though probably not.)  Whereas in reality, today, over <i>7 years</i> later, Python 2 is still more popular than Python 3 [1].<p>[1] <a href="https:&#x2F;&#x2F;lwn.net&#x2F;Articles&#x2F;640181&#x2F;" rel="nofollow">https:&#x2F;&#x2F;lwn.net&#x2F;Articles&#x2F;640181&#x2F;</a>