Please see the attached source file,  the related data file and the log
file.

When run with

 python hksu_copy.py  check_su.dat

the following is the output (which is incorrect):

-*- mode: compilation; default-directory: "c:/Users/J V/Documents/gamsdir/"
-*-
Compilation started at Sun Jan  7 07:54:52

python hksu_copy.py check_su.dat
nedge, check_k 1 1
nedge, check_k 2 2
nedge, check_k 3 4
nedge, check_k 4 2
nedge, klist 4 [1, 2, 4]
nedge, check_k 5 4
nedge, check_k 6 7
nedge, check_k 7 2
nedge, klist 7 [2, 4, 7]
nedge, check_k 8 4
nedge, check_k 9 7
nedge, check_k 10 1
nedge, klist 10 [2, 4, 7]
nedge, check_k 11 2
nedge, check_k 12 2
nedge, klist 12 [1, 2]
nedge, check_k 13 6
nedge, check_k 14 2
nedge, klist 14 [2, 6]
nedge, check_k 15 4
nedge, check_k 16 6
nedge, check_k 17 8
nedge, check_k 18 2
nedge, klist 18 [2, 4, 6, 8]
nedge, check_k 19 3
nedge, check_k 20 3
nedge, klist 20 [2, 3]
nedge, check_k 21 4
nedge, check_k 22 2
nedge, klist 22 [3, 4]
nedge, check_k 23 4
nedge, check_k 24 1
nedge, klist 24 [2, 4]
nedge, check_k 25 4
nedge, check_k 26 8
nedge, check_k 27 9
nedge, check_k 28 1
nedge, klist 28 [1, 4, 8, 9]
nedge, check_k 29 2
nedge, check_k 30 4
nedge, check_k 31 9
nedge, check_k 32 2
nedge, klist 32 [1, 2, 4, 9]
nedge, check_k 33 4
nedge, check_k 34 8
nedge, check_k 35 3
nedge, klist 35 [2, 4, 8]
nedge, check_k 36 4
nedge, check_k 37 7
nedge, check_k 38 8
nedge, check_k 39 2
nedge, klist 39 [3, 4, 7, 8]
nedge, check_k 40 3
nedge, check_k 41 4
nedge, check_k 42 2
nedge, klist 42 [2, 3, 4]
nedge, check_k 43 4
nedge, check_k 44 7
nedge, check_k 45 8
nedge, check_k 46 1
nedge, klist 46 [2, 4, 7, 8]
nedge, check_k 47 3
nedge, check_k 48 4
nedge, check_k 49 5
nedge, check_k 50 7
nedge, check_k 51 2
nedge, klist 51 [1, 3, 4, 5, 7]
nedge, check_k 52 4
nedge, check_k 53 7
nedge, check_k 54 1
nedge, klist 54 [2, 4, 7]
nedge, check_k 55 2
nedge, check_k 56 5
nedge, check_k 57 1
nedge, klist 57 [1, 2, 5]
nedge, check_k 58 2
nedge, check_k 59 5
nedge, check_k 60 9
nedge, check_k 61 2
nedge, klist 61 [1, 2, 5, 9]
nedge, check_k 62 5
nedge, check_k 63 2
nedge, klist 63 [2, 5]
nedge, check_k 64 5
nedge, check_k 65 6
nedge, check_k 66 9
nedge, check_k 67 1
nedge, klist 67 [2, 5, 6, 9]
nedge, check_k 68 5
nedge, check_k 69 1
nedge, klist 69 [1, 5]
nedge, check_k 70 3
nedge, check_k 71 5
nedge, check_k 72 9
nedge, check_k 73 5
nedge, klist 73 [1, 3, 5, 9]
nedge, check_k 74 7
nedge, check_k 75 9
nedge, check_k 76 1
nedge, klist 76 [5, 7, 9]
nedge, check_k 77 4
nedge, check_k 78 7
nedge, check_k 79 5
nedge, klist 79 [1, 4, 7]
nedge, check_k 80 5
nedge, klist 80 [5]
nedge, check_k 81 9
nedge, check_k 82 1
nedge, klist 82 [5, 9]
nedge, check_k 83 4
nedge, check_k 84 4
nedge, klist 84 [1, 4]
nedge, check_k 85 5
nedge, check_k 86 7
nedge, check_k 87 9
nedge, check_k 88 2
nedge, klist 88 [4, 5, 7, 9]
nedge, check_k 89 4
nedge, check_k 90 5
nedge, check_k 91 9
nedge, check_k 92 5
nedge, klist 92 [2, 4, 5, 9]
nedge, check_k 93 7
nedge, check_k 94 9
nedge, check_k 95 2
nedge, klist 95 [5, 7, 9]
nedge, check_k 96 5
nedge, check_k 97 9
nedge, check_k 98 4
nedge, klist 98 [2, 5, 9]
nedge, check_k 99 5
nedge, check_k 100 8
nedge, check_k 101 9
nedge, check_k 102 4
nedge, klist 102 [4, 5, 8, 9]
nedge, check_k 103 5
nedge, check_k 104 9
nedge, check_k 105 1
nedge, klist 105 [4, 5, 9]
nedge, check_k 106 3
nedge, check_k 107 9
nedge, check_k 108 1
nedge, klist 108 [1, 3, 9]
nedge, check_k 109 3
nedge, check_k 110 4
nedge, check_k 111 5
nedge, check_k 112 7
nedge, check_k 113 9
nedge, check_k 114 2
nedge, klist 114 [1, 3, 4, 5, 7, 9]
nedge, check_k 115 3
nedge, check_k 116 2
nedge, klist 116 [2, 3]
nedge, check_k 117 7
nedge, check_k 118 8
nedge, check_k 119 2
nedge, klist 119 [2, 7, 8]
nedge, check_k 120 3
nedge, check_k 121 7
nedge, check_k 122 1
nedge, klist 122 [2, 3, 7]
nedge, check_k 123 2
nedge, check_k 124 3
nedge, check_k 125 1
nedge, klist 125 [1, 2, 3]
nedge, check_k 126 2
nedge, check_k 127 4
nedge, check_k 128 5
nedge, check_k 129 1
nedge, klist 129 [1, 2, 4, 5]
nedge, check_k 130 2
nedge, check_k 131 3
nedge, check_k 132 4
nedge, check_k 133 5
nedge, check_k 134 3
nedge, klist 134 [1, 2, 3, 4, 5]
nedge, check_k 135 5
nedge, check_k 136 7
nedge, check_k 137 3
nedge, klist 137 [3, 5, 7]
nedge, check_k 138 5
nedge, check_k 139 7
nedge, klist 139 [3, 5]
nedge, check_k 140 2
nedge, klist 140 [7]
nedge, check_k 141 3
nedge, check_k 142 5
nedge, check_k 143 6
nedge, check_k 144 2
nedge, klist 144 [2, 3, 5, 6]
nedge, check_k 145 6
nedge, check_k 146 4
nedge, klist 146 [2, 6]
nedge, check_k 147 3
nedge, klist 147 [4]
nedge, check_k 148 6
nedge, check_k 149 3
nedge, klist 149 [3, 6]
nedge, check_k 150 1
nedge, klist 150 [3]
nedge, check_k 151 3
nedge, check_k 152 4
nedge, check_k 153 6
 No. of edges:  153
The last of the klists : nedge, klist 153 [1, 3, 4, 6]
 No. of left vertices :  51  No. of right vertices :  51
x :  0  klist :  [1, 3, 4, 6]
x :  1  klist :  [1, 3, 4, 6]
x :  2  klist :  [1, 3, 4, 6]
x :  3  klist :  [1, 3, 4, 6]
x :  4  klist :  [1, 3, 4, 6]
x :  5  klist :  [1, 3, 4, 6]
x :  6  klist :  [1, 3, 4, 6]
x :  7  klist :  [1, 3, 4, 6]
x :  8  klist :  [1, 3, 4, 6]
x :  9  klist :  [1, 3, 4, 6]
x :  10  klist :  [1, 3, 4, 6]
x :  11  klist :  [1, 3, 4, 6]
x :  12  klist :  [1, 3, 4, 6]
x :  13  klist :  [1, 3, 4, 6]
x :  14  klist :  [1, 3, 4, 6]
x :  15  klist :  [1, 3, 4, 6]
x :  16  klist :  [1, 3, 4, 6]
x :  17  klist :  [1, 3, 4, 6]
x :  18  klist :  [1, 3, 4, 6]
x :  19  klist :  [1, 3, 4, 6]
x :  20  klist :  [1, 3, 4, 6]
x :  21  klist :  [1, 3, 4, 6]
x :  22  klist :  [1, 3, 4, 6]
x :  23  klist :  [1, 3, 4, 6]
x :  24  klist :  [1, 3, 4, 6]
x :  25  klist :  [1, 3, 4, 6]
x :  26  klist :  [1, 3, 4, 6]
x :  27  klist :  [1, 3, 4, 6]
x :  28  klist :  [1, 3, 4, 6]
x :  29  klist :  [1, 3, 4, 6]
x :  30  klist :  [1, 3, 4, 6]
x :  31  klist :  [1, 3, 4, 6]
x :  32  klist :  [1, 3, 4, 6]
x :  33  klist :  [1, 3, 4, 6]
x :  34  klist :  [1, 3, 4, 6]
x :  35  klist :  [1, 3, 4, 6]
x :  36  klist :  [1, 3, 4, 6]
x :  37  klist :  [1, 3, 4, 6]
x :  38  klist :  [1, 3, 4, 6]
x :  39  klist :  [1, 3, 4, 6]
x :  40  klist :  [1, 3, 4, 6]
x :  41  klist :  [1, 3, 4, 6]
x :  42  klist :  [1, 3, 4, 6]
x :  43  klist :  [1, 3, 4, 6]
x :  44  klist :  [1, 3, 4, 6]
x :  45  klist :  [1, 3, 4, 6]
x :  46  klist :  [1, 3, 4, 6]
x :  47  klist :  [1, 3, 4, 6]
x :  48  klist :  [1, 3, 4, 6]
x :  49  klist :  [1, 3, 4, 6]
x :  50  klist :  [1, 3, 4, 6]

Compilation finished at Sun Jan  7 07:54:52

=====================================

The correct  results are:

x: 0    klist: [1,2,4]
x: 1    klist  [2,4,7]
x:2     klist: [2,4,7]

etc.

It isn't clear to me what bug you are reporting here:

- What do you mean by "problem reading back from a list of lists"? What sort of problem? 

- What makes you think that there is a bug in the interpreter, rather than in your own code?

We aren't going to debug your code for you: if you have found a bug, please report the simplest code that shows the bug. You may find it helpful to read this first:

http://sscce.org/

Although it is written for Java, the same applies to Python code too.

I'm closing this ticket for now.

If you simplify your code to something that demonstrates a bug in the Python interpreter, rather than a bug in your own code, please re-open this ticket with a better description of what the problem is, and the shortest, simplest demonstration of that problem that you can come up with.

Thank you.

Thanks for your sensible advice to produce
    a short, self-contained and correct example.

        Attached is the input file (35 lines, 770 bytes) and log (14 lines)

    Description of the problem and bug:

    The data consists of 6 triplets.

        The code is to group the data according to the pair in each
        triplet.

    In the data, there are just two pairs [1,3] and [2,2]

        With the pair [1,3] the associated third item in the first
        three triplets are 1, 2, 4.

        (This is printed correctly in the first line of the output).

         The bug is seen on the 4th (non-blank) line:

         klists[0] should be [1,2,4] and not as shown.

         I sincerely hope that I am not wasting your time with a
         non-existent bug.

     J. Viswanathan.

On Sun, Jan 7, 2018 at 8:22 AM, Steven D'Aprano <report@bugs.python.org>
wrote:

>
> Steven D'Aprano <steve+python@pearwood.info> added the comment:
>
> It isn't clear to me what bug you are reporting here:
>
> - What do you mean by "problem reading back from a list of lists"? What
> sort of problem?
>
> - What makes you think that there is a bug in the interpreter, rather than
> in your own code?
>
> We aren't going to debug your code for you: if you have found a bug,
> please report the simplest code that shows the bug. You may find it helpful
> to read this first:
>
> http://sscce.org/
>
> Although it is written for Java, the same applies to Python code too.
>
> I'm closing this ticket for now.
>
> If you simplify your code to something that demonstrates a bug in the
> Python interpreter, rather than a bug in your own code, please re-open this
> ticket with a better description of what the problem is, and the shortest,
> simplest demonstration of that problem that you can come up with.
>
> Thank you.
>
> ----------
> nosy: +steven.daprano
> resolution:  -> not a bug
> stage:  -> resolved
> status: open -> closed
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue32508>
> _______________________________________
>