Skipping the sliding window

Question

Given [1,2,3,4,5,6,7,8,9,10], to get a sliding window of 3 items at a time to get:

[(1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 6), (5, 6, 7), (6, 7, 8), (7, 8, 9), (8, 9, 10)]

From https://stackoverflow.com/q/42220614/610569, a sliding window of a sequence can be achieved with:

def per_window(sequence, n=1):
    """
    Returns a sliding window.
    From https://stackoverflow.com/q/42220614/610569
        >>> list(per_window([1,2,3,4], n=2))
        [(1, 2), (2, 3), (3, 4)]
        >>> list(per_window([1,2,3,4], n=3))
        [(1, 2, 3), (2, 3, 4)]
    """
    start, stop = 0, n
    seq = list(sequence)
    while stop <= len(seq):
        yield tuple(seq[start:stop])
        start += 1
        stop += 1

But if there's some constraints that I want to put in the sliding window, and I only want to get the windows where a certain element exist.

Let's say I only want the windows that contains 4, I could something like:

>>> [window for window in per_window(x, 3) if 4 in window]
[((2, 3, 4), (3, 4, 5), (4,5,6)]

But somehow the loop still has to process through the whole list of windows and check through the if conditions.

I could do some skipping by looking for the positions of 4 and limit the input to the per_window, e.g.

# Input sequence.
x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
# Window size.
n = 3
# Constraint.
c = 4 
# Set the index to 0
i = 0
while i < len(x)-n:
    i = x.index(4, i)
    # First window where the constraint is met.
    left = i - (n-1)
    if left > 0:
        print (list(per_window(x[left:i], 3)))
    right = i + n
    if right < len(x):
        print (list(per_window(x[i:right], 3)))
    i = right

(Note the code above with the ifs don't work =( )

Instead of finding the index outside of the per_window function, is there another way to add such a constraint in the per_window function?

---

EDITED

After reading @RaymondHettinger's answer:

def skipping_window(sequence, target, n=3):
    """
    Return a sliding window with a constraint to check that
    target is inside the window.
    From https://stackoverflow.com/q/43626525/610569
    """
    start, stop = 0, n
    seq = list(sequence)
    while stop <= len(seq):
        subseq = seq[start:stop]
        if target in subseq:
            yield tuple(seq[start:stop])
        start += 1
        stop += 1
        # Fast forwarding the start.
        # Find the next window which contains the target.
        try:
            # `seq.index(target, start) - (n-1)` would be the next
            # window where the constraint is met.
            start = max(seq.index(target, start) - (n-1), start)
            stop = start + n
        except ValueError:
            break

[out]:

>>> x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> list(skipping_window(x, 4, 3))
[(2, 3, 4), (3, 4, 5), (4, 5, 6)]

Answer 1

Instead of finding the index outside of the per_window function, is there another way to add such a constraint in the per_window function?

Yes, you can add the condition just before the yield:

def per_window(sequence, target, n=1):
    start, stop = 0, n
    seq = list(sequence)
    while stop <= len(seq):
        subseq = seq[start:stop]
        if target in subseq:
            yield tuple(subseq)
        start += 1
        stop += 1