The question creates a very complicated dictionary, so complicated that even the keys are not easily accessible.

In the following code, i will also define a matrix `K`

, and an `n`

for my taste, in the same time using exact objects.

```
n = 3
K = matrix( ZZ, 3, 3, [ 1,2,1,2,1,2,1,2,1 ] )
import itertools
W = itertools.product( [0,1], repeat = n )
preimages = {}
for w in W:
w = vector(w)
w = w.row()
v = w.transpose()
u = w * K * v
u.set_immutable()
if u in preimages:
preimages[u].append(w)
else:
preimages[u] = [w]
keys = preimages.keys()
for key in keys:
print key, type(key), preimages[ key ]
# trying to get the value for the MATRIX [6], which is ("convinced" to be) a key...
m = matrix( ZZ, 1, 1, [6,] )
m.set_immutable()
print "preimages of [6,] is:", preimages[ m ]
```

This gives:

```
[0] <type 'sage.matrix.matrix_integer_dense.Matrix_integer_dense'> [[0 0 0]]
[1] <type 'sage.matrix.matrix_integer_dense.Matrix_integer_dense'> [[0 0 1], [0 1 0], [1 0 0]]
[13] <type 'sage.matrix.matrix_integer_dense.Matrix_integer_dense'> [[1 1 1]]
[6] <type 'sage.matrix.matrix_integer_dense.Matrix_integer_dense'> [[0 1 1], [1 1 0]]
[4] <type 'sage.matrix.matrix_integer_dense.Matrix_integer_dense'> [[1 0 1]]
preimages of [6,] is: [[0 1 1], [1 1 0]]
```

Note:

There are many other better ways to build versions of the above `preimages`

dictionary.
For instance:

```
n = 3
K = matrix( ZZ, 3, 3, [ 1,2,1,2,1,2,1,2,1 ] )
V = [ vector( c )
for c in cartesian_product( [ [ ZZ(0), ZZ(1) ] for _ in range(n) ] ) ]
def f( v ):
return ( v.row() * K * v.column() )[0,0]
f_values = list( set( [ f(v) for v in V ] ) )
preimages = dict( [ ( val, [ v for v in V if f(v) == val ] )
for val in f_values ] )
```

The generated dictionary is:

```
sage: import pprint
sage: pprint.pprint( preimages )
{0: [(0, 0, 0)],
1: [(0, 0, 1), (0, 1, 0), (1, 0, 0)],
4: [(1, 0, 1)],
6: [(0, 1, 1), (1, 1, 0)],
13: [(1, 1, 1)]}
```

Please provide your code with a correct Sage/Python syntax,

`[[0 0 0 0 0]]`

is meaningless, also, it is not possible to have a list as a key for a dictionary since it is unhashable, so having a dictionary whose definition starts with`{[0.0]:`

is meaningless too. As is, there is no hope we can reproduce anything.Please give code producing a simple dictionary (with two keys) so that the values do not lead to errors - note that

`[0 0 0 0 0]`

immediately gives an error.Why are the keys (displayed) above

lists(and how did this dictionary come into sage)? Why not simple elements? Or tuples if we really need one element tuples? For instance:Why do we use approximate values in the (tuples)? This may be a good reason to "miss the key", for instance the last one, if the precision does not match or if after computation an other value is delivered "not the expected one".

The code looks like this:

The problem is most likely because I have set u immutable, but I need to get preimages for my function u = w * K * v

if I change the last part of my code to:

(results are defines before as list or tuple of all u) the problem with keys seems to be solved, but I do not get appropriate vectors as preimages, instead I get (1, 1, 1, 1, 1) for all u

the best I can get, but the problem remains the same:

I think the problem is that u is a matrix, is there a way to convert it to a number ?