Print intermediate solutions (Objective value and variables) while running MIP using gurobi in python.

When running my gurobi optimization for 60 seconds. I see my objective function value below, is there a way for me to see what variables were used (I have 8) during each of these intermdediate steps aswell.

 Nodes    |    Current Node    |     Objective Bounds      |     Work
 Expl Unexpl |  Obj  Depth IntInf | Incumbent    BestBd   Gap | It/Node Time

     0     0 1.1957e+09    0  477 3.5443e+09 1.1957e+09  66.3%     -    0s
H    0     0                    2.477577e+09 1.1957e+09  51.7%     -    0s
H    0     0                    1.989941e+09 1.1957e+09  39.9%     -    0s
H    0     0                    1.805903e+09 1.1957e+09  33.8%     -    0s
H    0     0                    1.780393e+09 1.1957e+09  32.8%     -    0s
     0     0 1.2321e+09    0  529 1.7804e+09 1.2321e+09  30.8%     -    0s
H    0     0                    1.701134e+09 1.2321e+09  27.6%     -    0s
     0     0 1.2321e+09    0  523 1.7011e+09 1.2321e+09  27.6%     -    0s
     0     0 1.2321e+09    0  453 1.7011e+09 1.2321e+09  27.6%     -    1s
     0     0 1.2321e+09    0  453 1.7011e+09 1.2321e+09  27.6%     -    1s
     0     2 1.2321e+09    0  453 1.7011e+09 1.2321e+09  27.6%     -    1s
H   31    36                    1.699992e+09 1.3234e+09  22.2%   416    1s
H   34    36                    1.699210e+09 1.3234e+09  22.1%   395    1s
H   35    36                    1.697900e+09 1.3234e+09  22.1%   384    1s
H   62    76                    1.697222e+09 1.3234e+09  22.0%   232    1s
H   66    76                    1.697109e+09 1.3234e+09  22.0%   220    1s
H   70    76                    1.696988e+09 1.3234e+09  22.0%   208    1s
H   77    84                    1.695236e+09 1.3234e+09  21.9%   190    2s
H   83    84                    1.693595e+09 1.3234e+09  21.9%   177    2s
H  109   132                    1.693386e+09 1.3234e+09  21.9%   138    2s
H  112   132                    1.693081e+09 1.3234e+09  21.8%   135    2s
H  130   132                    1.692910e+09 1.3234e+09  21.8%   121    2s
H  165   186                    1.692692e+09 1.3234e+09  21.8%  99.4    2s
H  169   186                    1.692642e+09 1.3234e+09  21.8%  97.1    2s
H  172   186                    1.692554e+09 1.3234e+09  21.8%  95.6    2s
H  438   439                    1.692534e+09 1.3234e+09  21.8%  83.8    2s
H  439   439                    1.692480e+09 1.3234e+09  21.8%  85.0    2s
H  472   482                    1.686467e+09 1.3234e+09  21.5%  82.0    2s
H  497   482                    1.686150e+09 1.3234e+09  21.5%  80.3    2s
H 1508  1457                    1.685521e+09 1.3234e+09  21.5%  34.4    3s
H 1801  1709                    1.684923e+09 1.3234e+09  21.5%  30.8    3s
H 2367  2131                    1.682392e+09 1.3234e+09  21.3%  26.4    3s
H 2372  2112                    1.680475e+09 1.3234e+09  21.3%  26.4    3s
H 2377  2000                    1.678343e+09 1.3234e+09  21.2%  26.4    4s
  2379  2001 1.6106e+09  169  491 1.6783e+09 1.3234e+09  21.2%  26.3    5s
H 2556  2036                    1.669077e+09 1.3234e+09  20.7%  28.2    7s
H 2561  1939                    1.664964e+09 1.3234e+09  20.5%  28.5    7s
  3617  2530 1.3234e+09   49  542 1.6650e+09 1.3234e+09  20.5%  24.2   10s
H 3624  2437                    1.663481e+09 1.3234e+09  20.4%  24.2   10s
  7307  4984 1.5220e+09  138  604 1.6635e+09 1.3234e+09  20.4%  20.5   15s
 19236 15554 1.4508e+09   81  684 1.6635e+09 1.3234e+09  20.4%  19.3   20s
 35012 29523 1.5993e+09  405  450 1.6635e+09 1.3234e+09  20.4%  20.2   25s
 43252 35214 1.6122e+09  469 1784 1.6635e+09 1.3234e+09  20.4%  19.6   34s
 43255 35216 1.3698e+09   26  479 1.6635e+09 1.3234e+09  20.4%  19.6   35s
 43347 35289 1.6222e+09   30  671 1.6635e+09 1.6057e+09  3.47%  20.0   40s
H43359 33523                    1.663148e+09 1.6057e+09  3.46%  20.0   40s
H43465 31924                    1.662385e+09 1.6057e+09  3.41%  20.0   42s
 44481 32303 1.6400e+09   58  670 1.6624e+09 1.6057e+09  3.41%  20.2   45s
 49153 33472 1.6460e+09   66  893 1.6624e+09 1.6057e+09  3.41%  20.4   50s
 53305 35088 1.6057e+09   37  713 1.6624e+09 1.6057e+09  3.41%  21.1   55s
 57290 36547 1.6168e+09   83  679 1.6624e+09 1.6057e+09  3.41%  22.0   60s

Chung-Kyun Han

Gurobi Staff

July 13, 2023 00:29 Edited

Hi Bhargav Gorantla Sridhar,

Thank you for reaching us.

I would like to introduce two approaches that handle intermediate solutions.
The simple one is to write .sol file by SolFiles parameter and I found a post asking the similar question to yours.
Another approach is to use a callback function(by requesting the MIPSOL_SOL in a MIPSOL callback).
Please check the linked Python example if you want to know how to implement a callback function.

Here is an example python code dealing with a callback function and SOLFiles parameter.

#Print intermediate solutions - two ways 1) callback 2) parameter SolFiles

import gurobipy as gp
from gurobipy import GRB
 
def mycallback(model, where):

    if where == GRB.Callback.MIPSOL:

        # Access solution values using the custom attribute model._vars
        sol = model.cbGetSolution(model._vars)
        obj = model.cbGet(GRB.Callback.MIPSOL_OBJ)

        # Print the solution with variable names
        print('**** New solution of obj %g '%obj)
        for i in range(len(model._vars)):
           print(f"{model._vars[i].VarName}: {sol[i]}")

model = gp.Model()

'''
    Build your own model
'''

# Create custom attribute model._vars
model._vars = model.getVars()

#Alternative - you can also use the SolFiles parameter to write new solutions to disk.
model.params.SolFiles = "solution"

model.optimize(mycallback)

If you have further questions, feel free to contact us again.

Best regards,
Chung-Kyun Han

Print intermediate solutions (Objective value and variables) while running MIP using gurobi in python.

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