KeyError when running TSP example (tsp.ipynb) during m.optimize(subtourelim)

Hi everyone,

I'm trying to run the Traveling Salesman Problem (TSP) example notebook (tsp.ipynb) from the Gurobi examples, but I encountered an error on the line m.optimize(subtourelim):

KeyError                                  Traceback (most recent call last)

<ipython-input-8-1587b32d4cc3> in <cell line: 3>()
      1 m._vars = vars
      2 m.Params.lazyConstraints = 1
----> 3 m.optimize(subtourelim)

src/gurobipy/_model.pyx in gurobipy._model.Model.optimize()

src/gurobipy/_model.pyx in gurobipy._model.Model.update()

KeyError: 0

This error occurs both on my local machine (Windows) and on Google Colab. The environment details are as follows:

• Gurobi version: 12.0.0
• Python version: 3.9.13
• Operating system: Windows 10 (or 11, please specify)
• Google Colab: Error occurs on Colab as well

I have not modified the original tsp.ipynb example code except for providing the callback function, which is exactly the same as the one in the original example. Here’s the callback function I am using:

# Callback - use lazy constraints to eliminate sub-tours
def subtourelim(model, where):
    if where == GRB.Callback.MIPSOL:
        print(111)
        # make a list of edges selected in the solution
        vals = model.cbGetSolution(model._vars)
        selected = gp.tuplelist((i, j) for i, j in model._vars.keys()
                             if vals[i, j] > 0.5)
        # find the shortest cycle in the selected edge list
        tour = subtour(selected)
        if len(tour) < len(capitals):
            # add subtour elimination constr. for every pair of cities in subtour
            model.cbLazy(gp.quicksum(model._vars[i, j] for i, j in combinations(tour, 2))
                         <= len(tour)-1)

# Given a tuplelist of edges, find the shortest subtour
def subtour(edges):
    unvisited = capitals[:]
    cycle = capitals[:] # Dummy - guaranteed to be replaced
    while unvisited:  # true if list is non-empty
        thiscycle = []
        neighbors = unvisited
        while neighbors:
            current = neighbors[0]
            thiscycle.append(current)
            unvisited.remove(current)
            neighbors = [j for i, j in edges.select(current, '*')
                         if j in unvisited]
        if len(thiscycle) <= len(cycle):
            cycle = thiscycle # New shortest subtour
    return cycle

Additional Details:

• Callback Function: The callback function is exactly the same as in the original example.
• Issue: The error occurs when I call m.optimize(subtourelim), and both my local machine and Google Colab report the same KeyError: 0.

I suspect the issue may be related to how variables are indexed or passed to the callback function, or possibly an issue with Gurobi's configuration. Does anyone know how to resolve this error or have any suggestions?

Thank you in advance for your help!