List comprehension as substitute for reduce() in Python

Question

The following python tutorial says that:

List comprehension is a complete substitute for the lambda function as well as the functions map(), filter() and reduce().

http://python-course.eu/python3_list_comprehension.php

However, it does not mention an example how a list comprehension can substitute a reduce() and I can't think of an example how it should be possible.

Can please someone explain how to achieve a reduce-like functionality with list comprehension or confirm that it isn't possible?

Answer 1

Ideally, list comprehension is to create a new list. Quoting official documentation,

List comprehensions provide a concise way to create lists. Common applications are to make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.

whereas reduce is used to reduce an iterable to a single value. Quoting functools.reduce,

Apply function of two arguments cumulatively to the items of sequence, from left to right, so as to reduce the sequence to a single value.

So, list comprehension cannot be used as a drop-in replacement for reduce.

Answer 2

I was surprised at first to find that Guido van Rossum, creator of Python, was against reduce. His reasoning was that beyond summing, multiplying, and-ing, and or-ing, using reduce yields an unreadable solution that is better suited by a function which iterates through and updates an accumulator. His article on the matter is here. So no, there isn't a list comprehension alternative to reduce, instead the "pythonic" way is to implement an accumulating function the old fashioned way:

Instead of:

out = reduce((lambda x,y: x*y),[1,2,3])

Use:

def prod(myList):     out = 1     for el in myList:         out *= el     return out 

Of course nothing stops you from continuing to use reduce (python 2) or functools.reduce (python 3)