Creating a Decision Variable in Python-MIP with Dictionaries

Question

I have already seen in the Python-MIP documentation on how to implement a variable with multiple indices. In each example I read, this is done with the use of lists and integers as indices.

This is how I implement a variable that I needed in the beginning:

x = [[[model.add_var(name='x({},{},{})'.format(e,s,a),var_type=INTEGER,ub = employeeDict[e].mitarbeiter_ist_formen)
   for e in employeeDict] for s in demandBelongingToShiftDict] for a in demandAppendedWithShiftDict]

But got the following error:

TypeError: list indices must be integers or slices, not str

Since the index a is a string.

Now I know that in Python is impossible to search within a list with strings as indices. This is why I would like to use dictionaries:

x = {{{model.add_var(name='x({},{},{})'.format(e,s,a),var_type=INTEGER,ub = employeeDict[e].mitarbeiter_ist_formen)
   for e in employeeDict} for s in demandBelongingToShiftDict} for a in demandAppendedWithShiftDict}

Unfortunately, I am getting the following error:

TypeError: unhashable type: 'set'

Answer 1

Haven't used python-mip but to create multi-index variables I usually do:

x = {
    (e, s, a): model.add_var(
        name="x({},{},{})".format(e, s, a),
        var_type=INTEGER,
        ub=employeeDict[e].mitarbeiter_ist_formen,
    )
    for e in employeeDict
    for s in demandBelongingToShiftDict
    for a in demandAppendedWithShiftDict
}

and access it by x[e, s, a].

You are doing set instead of dict comprehensions because you are not creating key-value pairs.