No it doesn't, and that heuristic isn't possible. It's based on looking at the reference counting and mutating an immutable object. It's pretty awful.

If GP is referring to CPython patch #980695, it appears to be available in PyPy, but requires an additional flag.

"We have it, just not enabled by default. --objspace-with-strbuf I think" [1]

[1] https://mail.python.org/pipermail/python-dev/2013-February/1...