# How to split a list into pairs in all possible ways

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I have a list (say 6 elements for simplicity)

``````L = [0, 1, 2, 3, 4, 5]
``````

and I want to chunk it into pairs in ALL possible ways. I show some configurations:

``````[(0, 1), (2, 3), (4, 5)]
[(0, 1), (2, 4), (3, 5)]
[(0, 1), (2, 5), (3, 4)]
``````

and so on. Here `(a, b) = (b, a)` and the order of pairs is not important i.e.

``````[(0, 1), (2, 3), (4, 5)] = [(0, 1), (4, 5), (2, 3)]
``````

The total number of such configurations is `1*3*5*...*(N-1)` where `N` is the length of my list.

How can I write a generator in Python that gives me all possible configurations for an arbitrary `N`?

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## How to split a list into pairs in all possible ways split: Questions

How do you split a list into evenly sized chunks?

By jespern

I have a list of arbitrary length, and I need to split it up into equal size chunks and operate on it. There are some obvious ways to do this, like keeping a counter and two lists, and when the second list fills up, add it to the first list and empty the second list for the next round of data, but this is potentially extremely expensive.

I was wondering if anyone had a good solution to this for lists of any length, e.g. using generators.

I was looking for something useful in `itertools` but I couldn"t find anything obviously useful. Might"ve missed it, though.

2632

Here"s a generator that yields the chunks you want:

``````def chunks(lst, n):
"""Yield successive n-sized chunks from lst."""
for i in range(0, len(lst), n):
yield lst[i:i + n]
``````

``````import pprint
pprint.pprint(list(chunks(range(10, 75), 10)))
[[10, 11, 12, 13, 14, 15, 16, 17, 18, 19],
[20, 21, 22, 23, 24, 25, 26, 27, 28, 29],
[30, 31, 32, 33, 34, 35, 36, 37, 38, 39],
[40, 41, 42, 43, 44, 45, 46, 47, 48, 49],
[50, 51, 52, 53, 54, 55, 56, 57, 58, 59],
[60, 61, 62, 63, 64, 65, 66, 67, 68, 69],
[70, 71, 72, 73, 74]]
``````

If you"re using Python 2, you should use `xrange()` instead of `range()`:

``````def chunks(lst, n):
"""Yield successive n-sized chunks from lst."""
for i in xrange(0, len(lst), n):
yield lst[i:i + n]
``````

Also you can simply use list comprehension instead of writing a function, though it"s a good idea to encapsulate operations like this in named functions so that your code is easier to understand. Python 3:

``````[lst[i:i + n] for i in range(0, len(lst), n)]
``````

Python 2 version:

``````[lst[i:i + n] for i in xrange(0, len(lst), n)]
``````

2632

If you want something super simple:

``````def chunks(l, n):
n = max(1, n)
return (l[i:i+n] for i in range(0, len(l), n))
``````

Use `xrange()` instead of `range()` in the case of Python 2.x

2632

Directly from the (old) Python documentation (recipes for itertools):

``````from itertools import izip, chain, repeat

"grouper(3, "abcdefg", "x") --> ("a","b","c"), ("d","e","f"), ("g","x","x")"
``````

The current version, as suggested by J.F.Sebastian:

``````#from itertools import izip_longest as zip_longest # for Python 2.x
from itertools import zip_longest # for Python 3.x
#from six.moves import zip_longest # for both (uses the six compat library)

"grouper(3, "abcdefg", "x") --> ("a","b","c"), ("d","e","f"), ("g","x","x")"
``````

I guess Guido"s time machine works‚Äîworked‚Äîwill work‚Äîwill have worked‚Äîwas working again.

These solutions work because `[iter(iterable)]*n` (or the equivalent in the earlier version) creates one iterator, repeated `n` times in the list. `izip_longest` then effectively performs a round-robin of "each" iterator; because this is the same iterator, it is advanced by each such call, resulting in each such zip-roundrobin generating one tuple of `n` items.

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