Your example code doesn't behave the way you claim. my_list isn't changed, and `a` is a chain generator, not a list (without a further list wrapping).

In any event, there is no reason to involve reduce here. chain already handles varargs what you're trying to do without involving reduce at all:

a = list(itertools.chain(*my_list))

or if you prefer to avoid unnecessary unpacking:

a = list(itertools.chain.from_iterable(*my_list))

Either way, a will be [1, 2, 3, 4], and my_list will be unchanged, with no wasteful use of reduce.