Not possible to do (a, b) += (1, 2) in python?

Question

The following line doesn't seem to work:

(count, total) += self._GetNumberOfNonZeroActions((state[0] + x, state[1] - ring, state[2]))

I guess it is not possible to use the += operator in this case. I wonder why?

edit: Actually what I want is to add to variables count and total the values given by the tuple returned by that function. Now that I think of it, it doesn't make sense to allow (a, b) += (1, 2), as it would be creating a new tuple, right?

In other words, s there a way to simplify this?

    res = self._GetNumberOfNonZeroActions((state[0] + x, state[1] + ring, state[2]))
    count, total = res[0], res[1]

After reading your comments in response to answers, and now re-reading your question, I'm very confused about your actual goal. Your explanation doesn't include a loop, are you looping over different states and accumulating? Or do you just want tuple unpacking? Your "In other words" looks like you just want tuple unpacking, which could be achieved by replacing the `+=` in your original line with `=`. Looping over += with tuples will create a longer and longer tuple. — Jeffrey Harris, Dec 09 '09 at 16:09
I want to achieve something like point addition, this is (a, b) = (a, b) + (x, y) = (a+x, b+y). — devoured elysium, Dec 09 '09 at 18:01
In that case, your question is basically the same as http://stackoverflow.com/questions/497885/python-tuple-operations, the stuple solution there is perhaps less effort than depending on numpy. — Jeffrey Harris, Dec 10 '09 at 16:10

score 10 · Answer 1 · answered Dec 09 '09 at 05:39

Your observation is right: a += b for any a and b means the same as a = a + b (except that it may save one evaluation of a). So if a is a tuple, the only thing that can be +='d to it is another tuple; if a is a temporary unnamed tuple, that += will of course be unobservable -- Python helps you out by catching that special case as a syntax error. You need to give the tuple a name (or other reassignable reference) before you += to it...:

>>> thetup = (a, b)
>>> thetup += (1, 2)
>>> thetup
(23, 45, 1, 2)

if the syntax (a, b) += (1, 2) was accepted, it would of course have to imply the same functionality... but without any possible observable way to check that the appending had actually occurred, which really makes just about no sense. Good thing the syntax is NOT accepted, therefore!-)

That was it. I'm using data = (0,0) and data += ... and it works. Thanks! — devoured elysium, Dec 09 '09 at 05:43

score 5 · Answer 2 · answered Dec 09 '09 at 05:37

5

You are mixing two concepts together. Python supports tuple unpacking which allows you to assign more that one variable in a single line.

The += operator gets expanded by the interpreter since it is only a shorthand. Your example ((a, b) += (1, 2)) would be expanded to this:

(a, b) = (a, b) + (1, 2)

which, when you look at it, doesn't make a whole lot of sense. Just remember that tuple unpacking only works for the assignment of values to variables.

answered Dec 09 '09 at 05:37

Andrew Hare

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Yes, you are right. But my 1st example with (count, total) would make sense. – devoured elysium Dec 09 '09 at 05:39

score 1 · Accepted Answer · edited Dec 30 '22 at 19:01

1

If you want to work on numeric arrays, I recommend using numpy <Link>.

It allows you to do this:

>>> from numpy import *
>>> count_total = array((0,0))
>>> count_total += (1,2)
>>> count_total
array([1, 2])

edited Dec 30 '22 at 19:01

Glorfindel

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answered Dec 09 '09 at 10:00

Asmo Soinio

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Not possible to do (a, b) += (1, 2) in python?

3 Answers3