> But is the policy of CPython to care about alternative interpreter implementation?

I think that everybody would like to see more working implementations of Python:
https://hub.packtpub.com/lukasz-langa-at-pylondinium19-if-python-stays-synonymous-with-cpython-for-too-long-well-be-in-big-trouble/

That's why many tests are decorated with @support.cpython_only.

That's also why the PEP 399 exists.


> So if the new alternative implementation request to modify CPython code. Is it okay? If so I agree with not to remove Jython code on Python3.x

I'm not sure of which kind of changes do you have in mind.

Ronan Lamy who works on PyPy recently contributed to CPython Lib/stat.py module to respect the PEP 399: bpo-38109. Such contribution is very welcomed :-)

When PEPs are discussed, we also try to keep in mind that it should be possible to implement the change on other implementations. If it's not, it should be clearly mentioned and justified in the PEP.

Examples:

https://www.python.org/dev/peps/pep-0509/#changes

"The choice of increasing or not the version when a dictionary method does not change its content is left to the Python implementation. A Python implementation can decide to not increase the version to avoid dictionary lookups in guards."

https://www.python.org/dev/peps/pep-0445/

"The only implementation required to conform to this PEP is CPython, but other implementations may choose to be compatible, or to re-use a similar scheme."

https://www.python.org/dev/peps/pep-0454/

"The tracemalloc module has been written for CPython. Other implementations of Python may not be able to provide it."