I'm facing this issue in python 3.6.5

How to reproduce:

my_list = [[1, 2], [3, 4]]
a = reduce(itertools.chain, my_list)

The output of `a' is [1, 2, 3, 4] as expected, but now 
my_list is [[1, 2, 3, 4], [3, 4]]

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.

You are right. It works now. I'm sorry for wasting your time.

Blech. Copy'n'paste error in last post:

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

should be:

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

(Note removal of *, which is the whole point of from_iterable)