How to return value of variables in python Gurobi interface

Answered

Amogh Bhosekar

• February 13, 2020 02:45

Hi,

I have a MIP problem coded in python. Following is a code for illustration

def problem():

1. m = Model()
2. add variables : x = {}
3. add constraints: ax <=b
4. add objective: min cTx
5. m.optimize()
6. return x

Next, I call the function

xvalue = problem()

1. print(xvalue) #print command 2

Now xvalue does not give me the value of the variable. 

I get the following (1, 1, 1, 1): <gurobi.Var *Awaiting Model Update*>,  Can anyone help me with this?

1

• Official comment

  

  Simranjit Kaur

  Gurobi Staff

  • December 06, 2024 00:49

  This post is more than three 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 why not try our AI Gurobot?.
• 

  Matthias Miltenberger

  Gurobi Staff

  • February 13, 2020 17:22

  Gurobi does not store all data that belongs to a variable inside the Python Var object. You need to have the Model object still around for querying solution data for example. In your code, though, everything related to the Model is only available within the function scope but not outside.

  A possible solution would be to pass the solution values along with the variable names in a dictionary:

  
  

  import gurobipy as gp

  from gurobipy import GRB

  
  

  def opti():

      m = gp.Model("mip1")

      x = m.addVar(name="x")

      y = m.addVar(name="y")

      z = m.addVar(name="z")

      m.setObjective(x + y + 2 * z, GRB.MAXIMIZE)

      m.addConstr(x + 2 * y + 3 * z <= 4, "c0")

      m.addConstr(x + y >= 1, "c1")

      m.optimize()

      return {x.varName: x.X}

  
  

  print(opti())

  1
• 

  Matthias Miltenberger

  Gurobi Staff

  • February 14, 2020 07:40

  Well, then you can change the last line to:

  solvals = opti()

  The main point here is that you need to store the values you need in some other form because you cannot access them outside the scope where Model exists.

  1
• 

  Matthias Miltenberger

  Gurobi Staff

  • February 13, 2020 17:25

  I deleted the previous comments for better readability.

  Cheers,
  Matthias

  0
• 

  Amogh Bhosekar

  • February 13, 2020 17:28

  

  

  

  Hello,

  I was not clear with my objective. I am not looking to print those in a function. I need to use the value of variables for next iteration. 

  I hope now my objective is clear. 

  Amogh

  0

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