How do I make demand_callback consider a variable that depends on to_node in ortools?

Question

I'm solving a vehicle routing problem using google ortools, and i need the demand_calback function take into consideration the time_visits (that depends on from_node and it's working) and the displacement between two nodes (that depends on from_node and to_node).

The problem is that the demand_callback function doesn't accept another parameter besides from_node, so i can't call to_node (and as consequence i can't consider the displacement time).

My code is:

# Add Capacity constraint.
def demand_callback(from_index):
   """Returns the demand of the node."""
   # Convert from routing variable Index to demands NodeIndex.
   from_node = manager.IndexToNode(from_index)
   '''Creates callback to get demands at each location.'''
   _time_visits = data['time_visits']
   return _time_visits[from_node] 

And I would like the code to look like this:

 def demand_callback(from_index, to_index):
    """Returns the demand of the node."""
    # Convert from routing variable Index to demands NodeIndex.
    from_node = manager.IndexToNode(from_index)
    to_node = manager.IndexToNode(to_index)

    '''Creates callback to get demands at each location.'''
    _time_visits = data['time_visits']
    _displacements = data['matrix_displacement'][from_node]

    result = _time_visits[from_node] + _displacements[from_node][to_node]
    return result

Answer 1

In the example, for demand dimension we advertise the use of unary callback.
Here, it seems you want to use a regular callback...

def demand_callback(from_index, to_index):
    """Returns the demand of the node."""
    # Convert from routing variable Index to demands NodeIndex.
    from_node = manager.IndexToNode(from_index)
    to_node = manager.IndexToNode(to_index)

    '''Creates callback to get demands at each location.'''
    _time_visits = data['time_visits']
    _displacements = data['matrix_displacement'][from_node]

    result = _time_visits[from_node] + _displacements[from_node][to_node]
    return result

Then register it using RegisterTransitCallback() instead of RegisterUnaryTransitCallback().

demand_callback_index = routing.RegisterTransitCallback(demand_callback)