Repository: aimclub/GOLEM

Issue:
User-defined selections
For now [there is](https://github.com/aimclub/GOLEM/blob/main/golem/core/optimisers/genetic/operators/selection.py#L34C1-L37C80) no option to use custom selection operators. Such functionality could significantly expand the possibilities of using the framework.

Hints from issue comments:

Solution patch:
diff --git a/golem/core/optimisers/genetic/gp_params.py b/golem/core/optimisers/genetic/gp_params.py
index 7579dc9..b977fec 100644
--- a/golem/core/optimisers/genetic/gp_params.py
+++ b/golem/core/optimisers/genetic/gp_params.py
@@ -83,8 +83,7 @@ class GPAlgorithmParameters(AlgorithmParameters):
     adaptive_mutation_type: MutationAgentTypeEnum = MutationAgentTypeEnum.default
     context_agent_type: Union[ContextAgentTypeEnum, Callable] = ContextAgentTypeEnum.nodes_num
 
-    selection_types: Sequence[SelectionTypesEnum] = \
-        (SelectionTypesEnum.tournament,)
+    selection_types: Optional[Sequence[Union[SelectionTypesEnum, Any]]] = None
     crossover_types: Sequence[Union[CrossoverTypesEnum, Any]] = \
         (CrossoverTypesEnum.one_point,)
     mutation_types: Sequence[Union[MutationTypesEnum, Any]] = simple_mutation_set
@@ -96,7 +95,9 @@ class GPAlgorithmParameters(AlgorithmParameters):
     window_size: Optional[int] = None
 
     def __post_init__(self):
+        if not self.selection_types:
+            self.selection_types = (SelectionTypesEnum.spea2,) if self.multi_objective \
+                else (SelectionTypesEnum.tournament,)
         if self.multi_objective:
-            self.selection_types = (SelectionTypesEnum.spea2,)
             # TODO add possibility of using regularization in MO alg
             self.regularization_type = RegularizationTypesEnum.none
diff --git a/golem/core/optimisers/genetic/operators/selection.py b/golem/core/optimisers/genetic/operators/selection.py
index 14850a8..c2f1ab7 100644
--- a/golem/core/optimisers/genetic/operators/selection.py
+++ b/golem/core/optimisers/genetic/operators/selection.py
@@ -33,6 +33,8 @@ class Selection(Operator):
         }
         if selection_type in selections:
             return selections[selection_type]
+        elif isinstance(selection_type, Callable):
+            return selection_type
         else:
             raise ValueError(f'Required selection not found: {selection_type}')
 


Test patch:
diff --git a/test/unit/optimizers/gp_operators/test_selection.py b/test/unit/optimizers/gp_operators/test_selection.py
index b0a8e67..922fe03 100644
--- a/test/unit/optimizers/gp_operators/test_selection.py
+++ b/test/unit/optimizers/gp_operators/test_selection.py
@@ -1,9 +1,15 @@
 from golem.core.adapter import DirectAdapter
 from golem.core.optimisers.genetic.gp_params import GPAlgorithmParameters
+from golem.core.optimisers.genetic.operators.operator import PopulationT
 from golem.core.optimisers.genetic.operators.selection import Selection, SelectionTypesEnum, random_selection
 from golem.core.optimisers.opt_history_objects.individual import Individual
 from test.unit.optimizers.test_evaluation import get_objective
 from test.unit.utils import graph_first, graph_second, graph_third, graph_fourth, graph_fifth
+from random import sample
+
+
+def custom_selection(population: PopulationT, pop_size: int):
+    return sample(population, pop_size)
 
 
 def get_population():
@@ -56,3 +62,13 @@ def test_individuals_selection_equality_individuals():
     selected_individuals_ref = [str(ind) for ind in selected_individuals]
     assert (len(selected_individuals) == num_of_inds and
             len(set(selected_individuals_ref)) == 1)
+
+
+def test_custom_selection():
+    num_of_inds = 3
+    population = get_population()
+    requirements = GPAlgorithmParameters(selection_types=[custom_selection], pop_size=num_of_inds)
+    selection = Selection(requirements)
+    selected_individuals = selection(population)
+    assert (all([ind in population for ind in selected_individuals]) and
+            len(selected_individuals) == num_of_inds)