How Can I Efficiently Iterate Over Overlapping Pairs (or N-element Windows) of List Values in Python?

Iterating Over Overlapping Pairs of List Values

When iterating over a Python list, it's often necessary to access both the current and subsequent elements. While using the zip function to pair consecutive values is effective, there may be a more efficient approach.

Using the pairwise() Function

Python 3.8 provides the itertools.pairwise() function, which pairs successive elements of an iterable:

import itertools

def pairwise(iterable):
    "s -> (s0, s1), (s1, s2), (s2, s3), ..."
    a, b = itertools.tee(iterable)
    next(b, None)
    return zip(a, b)

This function creates two iterators, a and b, pointing to the first element of the input iterable. b is advanced one step, resulting in a pointing to the current element and b pointing to the next element. zip is then used to pair the elements from both iterators.

Example Usage:

the_list = ['a', 'b', 'c', 'd']

for current, next in pairwise(the_list):
    print(current, next)

# Output:
# a b
# b c
# c d

Caveats:

It's crucial to note that pairwise() functions by iterating over the iterable multiple times. This means that if one iterator advances significantly faster than others, the implementation may retain consumed elements in memory to ensure they're available to all iterators.

Other Options for N-element Windows

The pairwise() function can be extended to create windows of arbitrary sizes:

def n_wise(iterable, n):
    "s -> (s0, s1, ..., s(n-1)), (s1, s2, ..., s(n)), ..."
    iterators = itertools.tee(iterable, n)
    for i in range(1, n):
        next(iterators[i], None)
    return zip(*iterators)

For example, to iterate over triples in a list:

for triplet in n_wise(the_list, 3):
    print(*triplet)

# Output:
# a b c
# b c d

Conclusion:

While the traditional method of iterating over overlapping pairs using zip is viable, the pairwise() and n_wise functions offer a concise and efficient way to achieve the same result for windows of any size.

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