Harry Smith - 3 months ago 6x
Python Question

# Preserve list values during list-to-set conversion in Python

When one converts a list to a set in Python using the

`set()`
function, Python tries to simplify the set by removing repeated values from the list, as in the following example.

``````In [1]: list = [1,4,1]

In [2]: list
Out[2]: [1, 4, 1]

In [3]: set = set(list)

In [4]: set
Out[4]: {1, 4}
``````

This is inconvenient when using sets to calculate properties of the list and converting back, as in the following vector subtraction example.

``````In [1]: a = [1,1,1]

In [2]: a
Out[2]: [1, 1, 1]

In [3]: b = [1,2,3]

In [4]: b
Out[4]: [1, 2, 3]

In [5]: list( set(a) - set(b) )
Out[5]: []
``````

Clearly something went wrong: we were expecting the operation
`{1-1, 1-2, 1-3} = {0, -1, -2}`
. Instead, we have the following stored values.

``````In [6]: set(a)
Out[6]: {1}

In [7]: set(b)
Out[7]: {1, 2, 3}
``````

This means we are calculating
`{1-1, ?-2, ?-3} = ?`
, which cannot be done.

Is there a way to retain the structure and values of a list when converting to a set in order to avoid this problem?

Clearly I am not talking about subtracting lists: I am talking about subtracting sets.

I don't see the need for `set` in this case. You can just `zip` the lists and do a subtraction like this
``````>>> a = [1,1,1]