I am using a lazy constraint callback in a model that I created with docplex.mp, fairly similar to the Traveling Salesman example that you can find in the answer to this StackOverflow post. I noticed this message in the log:

Warning: Control callbacks may disable some MIP features.
Lazy constraint(s) or lazy constraint/branch callback is present.
Disabling dual reductions (CPX_PARAM_REDUCE) in presolve.
Disabling presolve reductions that prevent crushing forms (CPX_PARAM_PREREFORM).
Disabling repeat represolve because of lazy constraint/incumbent callback.

According to the CPLEX manual, there is a newer "generic callback" mechanism that "does not require disabling any MIP features." This makes me thing that my problem might reach its solution faster if I used generic callbacks instead of legacy callbacks. Is it possible to write and use generic callbacks in Python? I don't see anything in the docplex.mp manual about generic callbacks. What would it take to use generic callbacks with docplex.mp?

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Leo Singer
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#DecisionOptimization

Hello Leo,

Generic callbacks are not yet directly supported by DOcplex, but you can use them through the `cplex` attribute of a Model instance,
and use the index-based Python API (short answer).
I wrote a small sample code at

https://github.com/PhilippeCouronne/docplex_contribs/blob/master/docplex_contribs/src/docplex_gencb.py

to show how this works. In particular I am using code from `docplex.mp.callbacks.ModelCallbackMixin` to convert a DOcplex constraint to
what is expected by the index-based API. This is a temporary workaround, but you can still leverage generic callbacks if need be.

    Philippe.

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Philippe Couronne
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