cell_set.py

#!/usr/bin/env python

from platform import python_implementation
if python_implementation() not in ('CPython', 'PyPy'):
    raise NotImplementedError('unsupported')

from types import FunctionType
def cell_set(obj, val):
    def _(val, cell = None):
        def _(val):
            nonlocal cell
            cell = val
        return _
    _ = _(val)
    FunctionType(_.__code__, _.__globals__, _.__name__, _.__defaults__, (obj,))(val)

def f():
    a = 'ayy'
    def g():
        cell_set((lambda: a).__closure__[0], 'lmao')
    g()
    return a

assert f() == 'lmao'

Makefile

SHELL = /bin/zsh

.PHONY: test
.ONESHELL:
test: cell_set.py
	@for interp in pypy pypy3 python2 python lua; do
		printf "%-8s %s\n" "$$interp" "$$($$interp $^ >/dev/null 2>&1 && echo "✓" || echo "✗")"
	done