How to unpack a tuple from left to right?

Question

Is there a clean/simple way to unpack a Python tuple on the right hand side from left to right?

For example for

j = 1,2,3,4,5,6,7

(1,2,3,4,5,6,7)

v,b,n = j[4:7]

Can I modify the slice notation so that v = j[6], b=j[5], n=j[4] ?

I realise I can just order the left side to get the desired element but there might be instances where I would just want to unpack the tuple from left to right I think.

If you wish to reverse arbitrary sequences for assignment, `v,b,n = reversed(my_values)` will do. For example, a `collections.deque` cannot be reversed by slice notation. — MisterMiyagi, Jul 08 '16 at 23:11

score 51 · Answer 1 · answered Jul 08 '16 at 22:12

51

This should do:

v,b,n = j[6:3:-1]

A step value of -1 starting at 6

answered Jul 08 '16 at 22:12

Moses Koledoye

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Thanks so much that did it. I tried the same but as an absolute beginner had tried [3:6:-1] not understanding the start stop index needed to be reversed as well – Anthony Lee Meier Jul 09 '16 at 00:46
@AnthonyLeeMeier You can also have a look at this thread on [SO](http://stackoverflow.com/questions/509211/explain-pythons-slice-notation) – Moses Koledoye Jul 09 '16 at 01:15
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I think maybe `v, b, n = j[:-4:-1]` is better, since it's agnostic about the length of `j`. – Paul Jul 09 '16 at 13:09

Aguy · Accepted Answer · 2016-07-08T23:12:09.520

14

In case you want to keep the original indices (i.e. don't want to bother with changing 4 and 7 to 6 and 3) you can also use:

v, b, n = (j[4:7][::-1])

edited Jul 08 '16 at 23:12

answered Jul 08 '16 at 23:04

Aguy

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This works as well thank you for your help. The index of the last element with value 7 is [6] and in yours we range from [4] inclusive to [7] not included. In Moses' answer I think it is similar but ranges from [6] inclusive down to [3] not included. So it seems they both access the same indices and the solution provided by Mosses a bit cleaner I think. Thanks – Anthony Lee Meier Jul 09 '16 at 01:07

score 10 · Answer 3 · answered Jul 08 '16 at 22:28

10

 n,b,v=j[4:7]

will also work. You can just change the order or the returned unpacked values

answered Jul 08 '16 at 22:28

joel goldstick

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*I realise I can just order the left side to get the desired element* – Padraic Cunningham Jul 08 '16 at 22:59

Padraic Cunningham · Answer 4 · 2016-07-08T22:57:54.833

9

You could ignore the first after reversing and use extended iterable unpacking:

j = 1, 2, 3, 4, 5, 6, 7


_, v, b, n, *_ = reversed(j)

print(v, b, n)

Which would give you:

6 5 4

Or if you want to get arbitrary elements you could use operator.itemgetter:

j = 1, 2, 3, 4, 5, 6, 7

from operator import itemgetter


def unpack(it, *args):
    return itemgetter(*args)(it)

v,b,n = unpack(j, -2,-3,-4)

print(v, b, n)

The advantage of itemgetter is it will work on any iterable and the elements don't have to be consecutive.

edited Jul 08 '16 at 22:57

answered Jul 08 '16 at 22:46

Padraic Cunningham

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Well, you can also do `v, b, n = reversed(j[4:7])` without using extended unpacking. this has the benefit that if `j` is long 10 million items the interpreter doesn't have to create new list containing all the other elements. – Bakuriu Jul 09 '16 at 09:22

score 0 · Answer 5 · answered Aug 22 '17 at 11:21

0

you can try this,

-1 mean looking from reverse, with last 3rd element to last element

>>> v,b,n=j[-1:-4:-1]
>>> print 'v=',v,'b=',b,'n=',n
v= 7 b= 6 n= 5
>>>

answered Aug 22 '17 at 11:21

Mohideen bin Mohammed

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How to unpack a tuple from left to right?

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