BESTSOLFOUND event handler on PySCIPopt

I am working on the problem in which I am willing to check and test some of the SCIP's event handlers. I see this SCIP document page and have some questions:

Do the event handlers callbacks return with something that may be printable? or just for assertation?
What exactly is the difference between SCIPincludeEventhdlr() and SCIPincludeEventhdlrBasic()?
When we really need to use an additional callback besides an event handler?

I tried to call an execute event handler to get the BESTSOLFOUND thought the solving process, but I got the following error:

class MyEvent(Eventhdlr):

    def eventinit(self):
        calls.append('eventinit')
        self.model.catchEvent(SCIP_EVENTTYPE.BESTSOLFOUND, self)

    def eventexit(self):
        calls.append('eventexit')
        self.model.dropEvent(SCIP_EVENTTYPE.BESTSOLFOUND, self)

    def eventexec(self, event):
        calls.append('eventexec')
        self.model.eventexec(SCIP_EVENTTYPE.BESTSOLFOUND, self)

AttributeError                            Traceback (most recent call last)
<ipython-input-11-c33e5174ab7e> in eventexec(self, event)
     16     def eventexec(self, event):
     17         calls.append('eventexec')
---> 18         self.model.eventexec(SCIP_EVENTTYPE.BESTSOLFOUND, self)
     19 
     20 

AttributeError: 'pyscipopt.scip.Model' object has no attribute 'eventexec'

Also, an MRE could be found here. I am wondering where am doing wrong and how can I fix that?

Just updated code:

import pytest
from pyscipopt import Model, Eventhdlr, SCIP_RESULT, SCIP_EVENTTYPE, SCIP_PARAMSETTING

calls = []

class MyEvent(Eventhdlr):

    def eventinit(self):
        calls.append('eventinit')
        self.model.catchEvent(SCIP_EVENTTYPE.BESTSOLFOUND, self)

    def eventexit(self):
        calls.append('eventexit')
        self.model.dropEvent(SCIP_EVENTTYPE.BESTSOLFOUND, self)

    def eventexec(self, event):
         calls.append('eventexec')


def test_event():
    # create solver instance
    s = Model()
    s.redirectOutput()
    s.printVersion()
    print("--------------------------------------")

    s.setPresolve(SCIP_PARAMSETTING.OFF)
    s.setSeparating(SCIP_PARAMSETTING.OFF)
    s.setHeuristics(SCIP_PARAMSETTING.OFF)

    eventhdlr = MyEvent()
    s.includeEventhdlr(eventhdlr, "BESTSOLFOUND", "python event handler to catch BESTSOLFOUND")

    # add some variables
    x = s.addVar("x", vtype="I")
    y = s.addVar("y", vtype="I")

    # add some constraint
    s.setObjective(7*x + 9*y, "maximize")
    s.addCons(-x + 3*y <=  6)
    s.addCons(7*x +  y <=  35)

    # solve problem
    s.optimize()

    # print solution
    print("--------------------------------------")
    print("x = ", s.getVal(x))
    print("y = ", s.getVal(y))

    del s

    assert 'eventinit' in calls
    assert 'eventexit' in calls
    assert 'eventexec' in calls

if __name__ == "__main__":
    test_event()


SCIP version 8.0.3 [precision: 8 byte] [memory: block] [mode: optimized] [LP solver: Soplex 6.0.3] [GitHash: 62fab8a2e3]
Copyright (C) 2002-2022 Konrad-Zuse-Zentrum fuer Informationstechnik Berlin (ZIB)
--------------------------------------
presolving:
   (0.0s) symmetry computation started: requiring (bin +, int +, cont +), (fixed: bin -, int -, cont -)
   (0.0s) no symmetry present
presolving (0 rounds: 0 fast, 0 medium, 0 exhaustive):
 0 deleted vars, 0 deleted constraints, 0 added constraints, 0 tightened bounds, 0 added holes, 0 changed sides, 0 changed coefficients
 0 implications, 0 cliques
presolved problem has 2 variables (0 bin, 2 int, 0 impl, 0 cont) and 2 constraints
      2 constraints of type <linear>
transformed objective value is always integral (scale: 1)
Presolving Time: 0.00

 time | node  | left  |LP iter|LP it/n|mem/heur|mdpt |vars |cons |rows |cuts |sepa|confs|strbr|  dualbound   | primalbound  |  gap   | compl. 
  0.0s|     1 |     0 |     1 |     - |   575k |   0 |   2 |   2 |   2 |   0 |  0 |   0 |   0 | 5.900000e+01 |      --      |    Inf | unknown
  0.0s|     1 |     2 |     1 |     - |   575k |   0 |   2 |   2 |   2 |   0 |  1 |   0 |   0 | 5.900000e+01 |      --      |    Inf | unknown
* 0.0s|     2 |     1 |     1 |   0.0 |    LP  |   1 |   2 |   2 |   2 |   0 |  1 |   0 |   0 | 5.500000e+01 | 3.500000e+01 |  57.14%| unknown
* 0.0s|     3 |     0 |     2 |   0.5 |    LP  |   1 |   2 |   1 |   2 |   0 |  1 |   0 |   0 | 5.500000e+01 | 5.500000e+01 |   0.00%|  58.64%

SCIP Status        : problem is solved [optimal solution found]
Solving Time (sec) : 0.01
Solving Nodes      : 3
Primal Bound       : +5.50000000000000e+01 (2 solutions)
Dual Bound         : +5.50000000000000e+01
Gap                : 0.00 %

Solution

I am quite sure you don't need the self.model.eventexec(SCIP_EVENTTYPE.BESTSOLFOUND, self) line in your exec callback. You just put the code you want to run there