How Can I Iterate Over Lists in Pairs or Groups in Python?

Pairwise Iteration Over Lists

Iterating over elements in pairs is a common requirement when manipulating lists. However, the standard for loops and list comprehensions in Python don't provide a built-in solution for pairwise traversal.

Pairwise Implementation

To overcome this limitation, a custom function called pairwise() can be implemented. This function takes an iterable as input and returns pairs of elements.

def pairwise(iterable):
    "s -> (s0, s1), (s2, s3), (s4, s5), ..."
    a = iter(iterable)
    return zip(a, a)

Usage

Using this pairwise() function, you can iterate over elements in pairs as follows:

l = [1, 2, 3, 4, 5, 6]

for x, y in pairwise(l):
    print("{} + {} = {}".format(x, y, x + y))

Output:

1 + 2 = 3
3 + 4 = 7
5 + 6 = 11

Generalized Grouping

For cases where you need to iterate over elements in groups of any size, a more generalized function called grouped() can be used.

def grouped(iterable, n):
    "s -> (s0,s1,s2,...sn-1), (sn,sn+1,sn+2,...s2n-1), (s2n,s2n+1,s2n+2,...s3n-1), ..."
    return zip(*[iter(iterable)] * n)

Usage

for x, y in grouped(l, 2):
    print("{} + {} = {}".format(x, y, x + y))

Type Checking with Mypy

For Python 3 users wishing to perform type checking with Mypy, the grouped() function can be annotated as follows:

from typing import Iterable, Tuple, TypeVar

T = TypeVar("T")

def grouped(iterable: Iterable[T], n=2) -> Iterable[Tuple[T, ...]]:
    """s -> (s0,s1,s2,...sn-1), (sn,sn+1,sn+2,...s2n-1), ..."""
    return zip(*[iter(iterable)] * n)

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