Documentation doesn't clearly state how selection operators behave with multiple objectives

[cmd-ntrf]

Reported by cgranade, Jun 4 (6 days ago) In the documentation posted at [0], it is not made clear which selection operators are single-objective, which ones treat the fitness vector in a lexographic fashion, and which are useful in MOEA contexts. For instance, selTournament seems to act lexographically, as it uses the sorted built-in with a tuple-valued fitness as the key, but this is not made clear in the documentation at [1].

Links: [0] http://deap.gel.ulaval.ca/doc/default/api/tools.html [1] http://deap.gel.ulaval.ca/doc/default/api/tools.html#deap.tools.selTournament

[kanosek]

The current documentation clearly suggests using weights to vary the importance of each objective one against another. This is very misleading with selection operators that treat the fitness vector in a lexographic fashion.

[blthayer]

Just stopping by to say I agree with @kanosek and the original filer of this ticket.

Take for example selBest vs. selTournament.

selBest uses sorted with the reverse=True argument. However, selTournament uses max.

This can lead to tricky behavior (note I'm using deap v1.3.0):

I would like to use deap to create a GA in which my weights represent, well, the weights of my objectives. All my weights will be negative, indicating I want the smallest possible value for each objective. So, I went ahead and subclassed base.Fitness like so to get around the lexicographical sorting (which doesn't meet my use case):

my_module.py:

from deap import base

class MultiMinFitness(base.Fitness):
    """Compare with sum of wvalues. Smaller (more negative) is better."""
    def __le__(self, other):
        return sum(self.wvalues) <= sum(other.values)

    def __lt__(self, other):
        return sum(self.wvalues) < sum(other.values)

Now, if I create some unittests to ensure my subclass is behaving correctly, we get inconsistent results between selBest and selTournament: test_my_module.py:

import unittest
from unittest.mock import patch

from .my_module import MultiMinFitness

from deap import creator, tools

class MultiFitnessTestCase(unittest.TestCase):

    @classmethod
    def setUpClass(cls):
        """Use the deap creator to set things up."""
        creator.create('FitnessMin', MultiMinFitness, weights=(-2, -4, -8))

    def test_selection(self):

        class MyClass:
            """Stub in a class to assign fitnesses to."""
            def __init__(self, f):
                self.fitness = f

            def __repr__(self):
                return str('MyClass: ' + str(self.fitness))

        # Initialize some fitness objects. Note that f2 is "more fit"
        # since the weighted sum is "more negative"
        f1 = creator.FitnessMin((1, 2, 3))
        f2 = creator.FitnessMin((2, 4, 6))

        # Create a couple class members.
        c1 = MyClass(f1)
        c2 = MyClass(f2)

        # Use selBest to ensure c2 (and thus f2) is best.
        best = tools.selBest([c1, c2], k=2)

        # c2 is the 'most negative' and therefore the best.
        self.assertIs(best[0], c2)  # Succeeds

        # Now perform a tournament selection. Patch selRandom so it just
        # returns our list of MyClass objects.
        with patch('deap.tools.selection.selRandom', return_value=[c1, c2]):
            best_tourn = tools.selTournament([c1, c2], k=1, tournsize=2)

        self.assertIs(best_tourn[0], c2)  # Fails.

Results of running test_my_module.py:

Failure
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/usr/local/lib/python3.7/unittest/case.py", line 615, in run
    testMethod()
  File "/opt/project/tmp/test_my_module.py", line 39, in test_selection
    self.assertIs(best_tourn[0], c2)  # Fails.
  File "/usr/local/lib/python3.7/unittest/case.py", line 1120, in assertIs
    self.fail(self._formatMessage(msg, standardMsg))
  File "/usr/local/lib/python3.7/unittest/case.py", line 680, in fail
    raise self.failureException(msg)
AssertionError: MyClass: (1.0, 2.0, 3.0) is not MyClass: (2.0, 4.0, 6.0)

[fmder]

Excellent point, thanks for your contribution. We'll consider this as part of the 2.0 refactoring that is underway.