In Python 3.3, itertools.accumulate()
, which normally repeatedly applies an addition operation to the supplied iterable, can now take a function argument as a parameter; this means it now overlaps with functools.reduce()
. With a cursory look, the main differences between the two now would seem to be:
accumulate()
defaults to summing but doesn't let you supply an extra initial condition explicitly whilereduce()
doesn't default to any method but does let you supply an initial condition for use with 1/0-element sequences, andaccumulate()
takes the iterable first whilereduce()
takes the function first.
Are there any other differences between the two? Or is this just a matter of behavior of two functions with initially distinct uses beginning to converge over time?
itertools.accumulate
is likereduce
but returns a generator* instead of a value. This generator can give you all the intermediate step values. So basically reduce gives you the last element of what accumulate will give you.*A generator is like an iterator but can be iterated over only once.
It seems that
accumulate
keeps the previous results, whereasreduce
(which is known as fold in other languages) does not necessarily.e.g.
list(accumulate([1,2,3], operator.plus))
would return[1,3,6]
whereas a plain fold would return6
Also (just for fun, don't do this) you can define
accumulate
in terms ofreduce
You can see in the documentation what the difference is.
reduce
returns a single result, the sum, product, etc., of the sequence.accumulate
returns an iterator over all the intermediate results. Basically,accumulate
returns an iterator over the results of each step of thereduce
operation.