addConstrs behaviour

Answered

Zvonimir Haramustek

• November 25, 2022 23:13

Hi,

I've been modeling a network flow optimization, and stumbled upon what appears to be a bug in the Python API.

for eq in lin_eqs.select('*', '*', '*'):
    self.model.addConstr(eq==0)

work just fine, but

self.model.addConstrs(
    (eq == 0 for eq in lin_eqs.select('*', '*', '*')), name="eq_is_zero")

throws

TypeError: unhashable type: 'gurobipy.LinExpr'

Could it be that addConstrs under the hood only accepts variables and not expressions?

I would have posted an minimum reproducible example but am in a bit of a rush, sorry about that. Hope you can reproduce it still.

Related to

• Python

1

• 

  Maliheh Aramon

  Gurobi Staff

  • November 26, 2022 01:46

  Hi Zvonimir, 

  As mentioned in our documentation, the Model.addConstrs() method returns a tupledict where the constraints are indexed by the values of the generator expression. In your code, I guess the \(\texttt{lin_eqs.select("*", "*", "*")}\) is a list of Gurobi LinExpr() objects which cannot be used as keys of the tupledict. One idea to fix this is given below:

  lin_eqs_list = lin_eqs.select('*', '*', '*')
  m.addConstrs((lin_eqs_list[i] == 0 for i in range(len(lin_eqs_list))), name="qe")

  Best regards,

  Maliheh

   

  2

Please sign in to leave a comment.