Issue with ConstrsGenerator

Hi,

I’m trying to implement lazy constraint generation by extending the ConstrsGenerator class. I’ve been following the docs here https://python-mip.readthedocs.io/en/latest/custom.html and the example here https://github.com/coin-or/python-mip/blob/master/examples/queens-lazy.py, but I can’t seem to get it to work.

I have this small toy example that is failing and I’m not sure why.

import mip

class DelayedConstraintGenerator(mip.ConstrsGenerator):
    def generate_constrs(self, model):
        print("generating_constraints")
        for v in model.vars:
            print(v, v.x)
            model += v >= 5

m = mip.Model()

x = m.add_var('x', var_type=mip.INTEGER)
y = m.add_var('y', var_type=mip.INTEGER)

m += x + y, "objective"
m += x + y <= 10

m.lazy_constrs_generator = DelayedConstraintGenerator()
status = m.optimize()

print(status)
for v in m.vars:
    print(v, v.x)

Output from the example is:

generating_constraints
x 0.0
y 0.0
generating_constraints
x 0.0
y 0.0
OptimizationStatus.INFEASIBLE
x None
y None

I’m on Python-MIP 1.6.2 and Python 3.6.9.

[h-g-s]

You’re welcome ! Thank @stefanhannie and @jooyous for the detailed bug report.

[h-g-s]

hi @jooyous and @stefanhannie , already fixed in master thanks to @jjhforrest ! will release another version tomorrow.