Thanks, that's clearer.

I'm still worried about the change in semantics where globals["__builtins__"] is assigned a different dict after the function object has been created (similar to https://bugs.python.org/file49816/func_builtins2.py).

I.e.

def foo(): return len("abc")
code = foo.__code__
g = {"__builtins__": {"len": len}}
f = FunctionType(code, g)
f()  # Succeeds
g["__builtins__"] = {}
f()  # Fails in 3.9 and before, passes in 3.10

Assuming code uses len, does f() succeed or fail?

I realize this is a pretty esoteric, but it does show the change in semantics (from later to earlier binding). Should we care? I like early binding because it allows more optimizations[1], but traditionally Python's semantics use late binding.

[1] Not in this case, the user could still change the meaning of len() with e.g.

g["__builtins__"]["len"] = lambda x: return 42