Getting the dual variable values when the model status is 4(INF_OR_UNBD)

Answered

Iason Liagkas

December 21, 2023 22:35

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Hello guys, I am running the benders algo, When my subproblem is feasible I am using the following line to get the dual variable value: 

gp.quicksum(BigM*deltars2kE[r,s,k]*subproblem.getConstrByName("UE_EV_2[E,{},{},{}]".format(r,s,k)).Pi for r,s in drs.keys() for k in K)

And it works. But when my subproblem becomes infeasible or unbounded and I want to take the dual variable values I am using the exact same code but with .FarkasDual instead of .Pi 

gp.quicksum(BigM*deltars2kE[r,s,k]*subproblem.getConstrByName("UE_EV_2[E,{},{},{}]".format(r,s,k)).FarkasDual for r,s in drs.keys() for k in K)

Then I get the following error:
AttributeError: Unable to retrieve attribute 'FarkasDual'.
Am I doing something wrong? Are there any specific conditions that the attribute FarkasDual works? 

 

Related to

• Sensitivity

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  November 13, 2026 01:32

  This post is more than two years old. Some information may not be up to date. For current information, please check the Gurobi Documentation or Knowledge Base. If you need more help, please create a new post in the community forum, or try Gurobot, our chatbot interface offering instant, expert-level support.
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  Eli Towle

  • Gurobi Staff

  December 21, 2023 23:41

  From the FarkasDual documentation:

  These attributes are only available when parameter InfUnbdInfo is set to 1.

  Have you set the InfUnbdInfo parameter to 1?

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  Iason Liagkas

  December 27, 2023 07:01

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  Thank you Eli, that was the problem, enjoy your holidays! 
  
  Kind regards
  Iason

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