A horrible context manager...

withlet.py

from contextlib import contextmanager
import sys

@contextmanager
def let(**kwargs):
    to_del = []
    to_set = {}
    bindings = sys._getframe(2).f_locals
    for k, v in kwargs.items():
        if k in bindings:
            to_set[k] = bindings[k]
        else:
            to_del.append(k)
        bindings[k] = v
    yield
    bindings.update(to_set)
    for k in to_del:
        del bindings[k]
    return False

a = 100
with let(a=1, b=2):
    print(a, b)
try:
    print(a)
    print(b)
except NameError as e:
    print(e)