fix: respect a zero mutation probability (#15)

Mutation now uses an explicit `is not None` check so a configured mutation probability of 0.0 is honoured instead of falling back to the default (the previous falsy check treated 0.0 as unset). Adds a regression test for the zero-probability case.

Author: alerodriargui

genetic_algorithm/chromosome.py

@@ -173,7 +173,7 @@ def __probability_mutation(self, probability):
 
     def mutate(self, method="probability_mutation", mutation_prob=None):
         if method == "probability_mutation":
-            mutation_prob = mutation_prob if mutation_prob else 1 / len(self.data)
+            mutation_prob = mutation_prob if mutation_prob is not None else 1 / len(self.data)
             return self.__probability_mutation(mutation_prob)
         elif method == "twors":
             return self.__twors_mutate()

tests/test_chromosome.py

@@ -55,6 +55,13 @@ def test_mutation_methods_preserve_length(method):
     assert len(out) == 6
 
 
+def test_probability_mutation_allows_zero_probability():
+    random.seed(1)
+    c = Chromosome(3, [(0, 0)] * 3, data=[1.0, 2.0, 3.0])
+    out = c.mutate(method="probability_mutation", mutation_prob=0)
+    assert out == [1.0, 2.0, 3.0]
+
+
 def test_permutation_mutations_preserve_multiset():
     c = Chromosome(5, [(0, 100)] * 5, data=[1.0, 2.0, 3.0, 4.0, 5.0])
     random.seed(2)