How to emulate sum() using a list comprehension?

No; a list comprehension produces a list that is just as long as its input. You will need one of Python's other functional tools (specifically reduce() in this case) to fold the sequence into a single value.

>>> from functools import reduce
>>> from operator import mul
>>> nums = [1, 2, 3]
>>> reduce(mul, nums)
6

Python 3 Hack

In regards to approaches such as [total := total + x for x in [1, 2, 3, 4, 5]]

This is a terrible idea. The general idea of emulate sum() using a list comprehension goes against the whole purpose of a list comprehension. You should not use a list comprehension in this case.

Python 2.5 / 2.6 Hack

In Python 2.5 / 2.6 You could use vars()['_[1]'] to refer to the list comprehension currently under construction. This is horrible and should never be used but it's the closest thing to what you mentioned in the question (using a list comp to emulate a product).

>>> nums = [1, 2, 3]
>>> [n * (vars()['_[1]'] or [1])[-1] for n in nums][-1]
6