can't find LP file

[HorizonLiang]

from pyscipopt import Model
model = Model("Example")

x = model.addVar("x")
y = model.addVar("y", vtype="INTEGER")
model.setObjective(x + y)
model.addCons(2*x - y*y >= 0)
model.optimize()

model.writeLP("./model.lp")

I can't find the LP file, while runing the code showing above.

[Joao-Dionisio]

Thank you for opening the issue! Apparently there's something wrong with writeLP(), since the code silently exits while trying to run it. So it isn't actually writing the LP, that's why you can't find it. Alternatively, you have writeProblem(), but I don't know if it does what you want...

[fserra]

yeah the issue is that writeLP writes the current LP, which mean, the current LP relaxation. If you look at the SCIP documentation, such a function can only be called in the solving stage (which is when an LP relaxation might exist). You need to use writeProblem as @Joao-Dionisio says.

[Joao-Dionisio]

Thanks, @fserra! There are quite a bit of issues related (even if indirectly) to PySCIPOpt's error handling, as referenced in Issue #610. I will try to take a look at it in the future.

[fserra]

yes, this is a bit annoying... I guess that ideally one would develop using the underlying scip in debug mode (then a lot more errors would pop up), but that might be too much to ask. An alternative is to start duplicating the checks that already exist in scip, e.g., add something like

        if not self.getStage() == SCIP_STAGE_SOLVING:
            raise Warning("method can only be called in solving stage")

I don't know what is worse

[HorizonLiang]

  model.writeProblem("model.cip")
  File "src/pyscipopt/scip.pyx", line 1408, in pyscipopt.scip.Model.writeProblem
  File "src/pyscipopt/scip.pyx", line 246, in pyscipopt.scip.PY_SCIP_CALL
OSError: SCIP: cannot create file!

thanks for your reply, however, it still show a error after using the function 'writeProblem'.
the error log shows above.

[Joao-Dionisio]

This is strange @HorizonLiang... I can run the code without issue:

Can it be a problem of having an outdated SCIP? Do you know which version you are using?

[Joao-Dionisio]

I am closing the issue to clear the Issues page, but feel free to reopen it if you still have problems!