for-loop over tuple in Python: ValueError

Question

I have a tuple of arrays like the following:

z = (array( [ [1], [2], [3]] ),
     array( [[10],[11],[12]] )
    )

I want to iterate over them with a simple for loop using two variables:

for x, y in z:
    print("it worked")
    self.doStuff(x, y)

... but it throws the error:

ValueError: too many values to unpack (expected 2)

I have googled this error. Many seem to suggest the .split() method or .items(). I'm not really sure, where to add those, since the don't belong to tuples. How can I execute this for-loop properly? Thanks for your help.

I assume you are trying to iterate over the arrays' elements, not over the tuple's elements, right? If so, you should re-think your data structures -- an iteration over a single 2-tuple unpacking 2 values results in a single run, where you unpack the 2 complete arrays from the tuple. — jbndlr
– jbndlr, Commented Sep 28, 2016 at 12:44

Philipp · Accepted Answer · 2016-09-28 12:29:44Z

2

Reading the other answer it might be that I misunderstood what you want.

You can also use

for x,y in zip(*z):

to unzip the z tuple.

The output then is:

it worked
[1] [10]
it worked
[2] [11]
it worked
[3] [12]

edited Sep 28, 2016 at 12:29

answered Sep 28, 2016 at 12:23

Philipp

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Ami Tavory · Accepted Answer · 2016-09-28 12:22:28Z

2

The line

for x, y in z:

assumes (at least in Python2.7), that each element in z can be tuple unpacked. This would be the case if each element in z is, say, a pair tuple:

In [23]: for x, y in [(1, 2), (3, 4)]:
    ...:     pass
    ...:

For your case, could it be that you simply want

x, y = z

?

This works by me:

In [19]: z = (array( [ [1], [2], [3]] ),
    ...:      array( [[10],[11],[12]] )
    ...:     )

In [20]: x, y = z

In [21]: x
Out[21]: 
array([[1],
       [2],
       [3]])

In [22]: y
Out[22]: 
array([[10],
       [11],
       [12]])

answered Sep 28, 2016 at 12:22

Ami Tavory

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