Variable object mapping issue in Python Callbacks with Presolve enabled

Hi,

The confusion with variable indices is a known issue in older versions of the Xpress Python interface. In later releases, accessing a variable object from a callback raises an error. This is because variable objects are associated with the original problem, but during a callback the problem is presolved, and Xpress does not allow access to the original problem data while the problem is presolved. The recommended way to access original variable properties during a callback is to cache the individual properties before the start of the solve, and then access e.g. self.variable_names['x'] instead of self.variables['x'].name from the callback.

If you need further help, please let me know which version of Xpress you are using, since the behaviour in this area has changed recently.