import os
from contextlib import contextmanager

@contextmanager
def temp_env(var: str, value: str | None):
    old = os.environ.get(var)
    try:
        if value is None:
            os.environ.pop(var, None)
        else:
            os.environ[var] = value
        yield
    finally:
        if old is None:
            os.environ.pop(var, None)
        else:
            os.environ[var] = old