model.NumVars doesn't work before solving model (in Gurobipy)?

Answered

Ben Farmer

May 20, 2025 06:09

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Here is one of the example problems built in Gurobipy, with some print statements added:

import gurobipy as gp
from gurobipy import GRB

# Create a new model
m = gp.Model("mip1")

# Create variables
x = m.addVar(vtype=GRB.BINARY, name="x")
y = m.addVar(vtype=GRB.BINARY, name="y")
z = m.addVar(vtype=GRB.BINARY, name="z")

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

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

# Add constraint: x + y >= 1
m.addConstr(x + y >= 1, "c1")

# Count number of variables
print("NumVars:", m.NumVars)

# Optimize model
m.optimize()

# Count number of variables again
print("NumVars:", m.NumVars)

The output is:

<license blah blah>
NumVars: 0
<solver output blah blah>
NumVars: 3

Is this the expected behaviour? I did not see anything in the docs for NumVars about this. And for my use case I really want to know things about the variables in the model *before* I solve it. Can this be achieved? It seems to work fine when I do a similar thing via the ortools API, but perhaps ortools is doing the counting there?

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• Python

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  Riley Clement

  • Gurobi Staff

  May 20, 2025 10:40

  Hi Ben,

  This is expected.  The relevant docs are here:

  https://docs.gurobi.com/projects/optimizer/en/current/reference/python/overview.html#lazy-updates

  You can use m.update() before calling m.NumVars in order to get the correct value.

  - Riley

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  Ben Farmer

  May 21, 2025 00:36

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  Ah, amazing, thanks very much!

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