How to work with zip on 2 generators s.t each returns the same item that changes its state overtime

Question

from collections import deque



def window(seq, n=2):

    it = iter(seq)

    win = deque((next(it, None) for _ in xrange(n)), maxlen=n)

    yield win

    append = win.append

    for e in it:

        append(e)

        yield win



a=[1,2,3]

b=[2,4,6]





for d in zip(window(a,2),window(b,2)):

    print d

    raw_input("Press Enter to continue...")

The output :

(deque([2, 3], maxlen=2), deque([4, 6], maxlen=2))

Press Enter to continue...

(deque([2, 3], maxlen=2), deque([4, 6], maxlen=2))

Press Enter to continue...

What im trying to do here is to capture all the zipped windows of length 2 of

a

and

b

, In other words im expecting the output to be like :

(deque([1, 2], maxlen=2), deque([2, 4], maxlen=2))

Press Enter to continue...

(deque([2, 3], maxlen=2), deque([4, 6], maxlen=2))

Press Enter to continue...

Clearly,

window

is yielding the same object over and over again only with a change in the items it holds, I guess what happens here is that

zip

is creating the iterable before the iteration starts so Im getting the same 2 deque objects zipped twice but the problem is that i get them in their final state.

How would you suggest to fix that without having to yield a copy in the window function?

For example changing window to return a copy would work:

def window(seq, n=2):

    it = iter(seq)

    win = deque((next(it, None) for _ in xrange(n)), maxlen=n)

    yield list(win)

    append = win.append

    for e in it:

        append(e)

        yield list(win)

But is less efficient, in my use case a and b are huge and ziping all those copies before the iteration is inefficient...

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Answer 1

To avoid zipping the two iterables before beginning the loop, use itertools.izip instead of zip in Python2:

import itertools as IT
import collections 

def window(seq, n=2):
    it = iter(seq)
    win = collections.deque((next(it, None) for _ in range(n)), maxlen=n)
    yield win
    append = win.append
    for e in it:
        append(e)
        yield win

a=[1,2,3]
b=[2,4,6]

for d in IT.izip(window(a,2),window(b,2)):
    print(d)

yields

[deque([1, 2], maxlen=2), deque([2, 4], maxlen=2)]
[deque([2, 3], maxlen=2), deque([4, 6], maxlen=2)]

In Python3, zip returns an iterator (identical to itertools.izip in Python2). This is why the code you posted already works in Python3 without changes.

How to work with zip on 2 generators s.t each returns the same item that changes its state overtime

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