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pythonattributeerrorgurobi

Callback function in gurobipy yields error messages

While working on my project, I need to change my MIP-model with a callback function. I followed the documentation closely, but I get always an error message.
I believe, the error comes with the cbGet-function, without this, I get an output, although I don't know whether it's correct.

Without the callback function, my model works just fine.

Here's my code:

import gurobipy as gb
...
some code with the model
...
def cb(mod, where):
    if where == gb.GRB.Callback.MIP:
        print(mod.cbGet(gb.GRB.Callback.MIP_NODCNT))
def cb2(mod, where):
    if where == gb.GRB.Callback.MIP:
        print(gb.GRB.Callback.MIP_NODCNT)
m.optimize(cb(m, gb.GRB.Callback.MIP))

Please note that if I use cb2 as callback function, I get the exit code 0 and an output. I'm not sure whether this output is right.
But when I'm using cb as callback function, I get the error message 

Traceback (most recent call last):
    File "C:/Users/dbigb/PycharmProjects/MA/MIP_model.py", line 39, in <module> m.optimize(cb(m, gb.GRB.Callback.MIP))
    File "C:/Users/dbigb/PycharmProjects/MA/MIP_model.py", line 32, in cb print(mod.cbGet(gb.GRB.Callback.MIP_NODCNT))
    File "model.pxi", line 4136, in gurobipy.Model.cbGet
    AttributeError: 'NoneType' object has no attribute '_cbdata'

I don't understand which attribute is none, neither what this _cbdata is or where I could find and check this. Any help would be appreciated.

I didn't include the whole model, since it's working fine. If it's important for the solution I will edit the question and add the code.

Solution

  • You should call optimize as follows:

    m.optimize(cb)

    The optimize method expects an argument of type function.

    In your code, cb(m, gb.GRB.Callback.MIP) would be evaluated (and become a constant) before starting the optimization.

    In the cb2 case, this simply prints 3002, i.e., the integer representation of gb.GRB.Callback.MIP_NODCNT. (See here.) The argument of optimize evaluates to None.

    In the cb case, the call to cbGet tries to access the model's callback, which is not defined. This is the NoneType in the error message.