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Runtime warning on what channel without breaking compatibility?

You could print the warning out to STDERR when the Python runtime finishes parsing and before it executes your script.

This will likely break some people's workflows, but honestly, it's the de-facto way of notifying users of a deprecation in many Python frameworks (Gunicorn did this recently with gunicorn-paster vs gunicorn --paste).

They could also include this in a "What's new in Python (4?)" since this is unlikely to be introduced in a minor version bump.

I'm not categorically against deprecation warnings. Isn't gunicorn a command line tool? I can respect such warnings in a tool or even sometimes a library, but not in a language interpreter. The user should as closely as possible be in full control of the streams in those cases, and any standard error output that isn't an exception (i.e. either deliberate or resulting from a programmer error) should stay the hell off.

The proper way to handle this is of course to introduce a new semver major version. No need for deprecation warnings because incompatibility is expected.

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