pyrimidine 1.5.4

A competitive framework for GA, designed by thorough OOP

pyrimidine

pyrimidine is an extensible framework of genetic/evolutionary algorithm by Python. See pyrimidine's document for more details.

Why

Why is the package named as “pyrimidine”? Because it begins with “py”.

— Are you kiding?

— No, I am serious.

Download

It has been uploaded to pypi, so download it with pip install pyrimidine, and also could download it from github.

Idea

We regard the population as a container of individuals, an individual as a container of chromosomes and a chromosome as a container(array) of genes.

The container could be a list or an array. The container class has an attribute element_class, telling itself the type of the elements in it.

Following is the part of the source code of BaseIndividual and BasePopulation.

class BaseIndividual(FitnessModel, metaclass=MetaContainer):
    element_class = BaseChromosome
    default_size = 1

class BasePopulation(PopulationModel, metaclass=MetaContainer):
    element_class = BaseIndividual
    default_size = 20

There is mainly tow kinds of containers: list and tuple as in programming language Haskell. See following examples.

# individual with chromosomes of type _Chromosome
_Individual1 = BaseIndividual[_Choromosome]
# individual with 2 chromosomes of type _Chromosome1 and _Chromosome2 respectively
_Individual2 = MixedIndividual[_Chromosome1, _Chromosome2]

Math expression

$s$ of type $S$ is a container of $a:A$, represented as follows:

s={a:A}:S

We define a population as a container of individuals or chromosomes, and an individual is a container of chromosomes.

Algebraically, an indivdiual has only one chromosome is equivalent to a chromosome mathematically. A population could also be a container of chromosomes. If the individual has only one chromosome, then just build the population based on chromosomes directly.

The methods are the functions or operators defined on $s$.

Use

Main classes

• BaseGene: the gene of chromosome
• BaseChromosome: sequence of genes, represents part of a solution
• BaseIndividual: sequence of chromosomes, represents a solution of a problem
• BasePopulation: a container of individuals, represents a container of a problem also the state of a stachostic process
• BaseMultipopulation: a container of population for more complicated optimalization

import

Just use the command from pyrimidine import * import all of the algorithms.

subclass

Chromosome

Generally, it is an array of genes.

As an array of 0-1s, BinaryChromosome is used most frequently.

Individual

just subclass MonoIndividual in most cases.

class MyIndividual(MonoIndividual):
    """individual with only one chromosome
    we set the gene is 0 or 1 in the chromosome
    """
    element_class = BinaryChromosome

    def _fitness(self):
        ...

Since the helper makeIndividual(n_chromosomes=1, size=8) could create such individual, it is equivalent to

class MyIndividual(binaryIndividual()):
    # only need define the fitness
    def _fitness(self):
        ...

If an individual contains several chromosomes, then subclass MultiIndividual or PolyIndividual. It could be applied in multi-real-variable optimization problems where each variable has a separative binary encoding.

In most cases, we have to decode chromosomes to real numbers.

class _Chromosome(BinaryChromosome):
    def decode(self):
        """Decode a binary chromosome
        
        if the sequence of 0-1 represents a real number, then overide the method
        to transform it to a nubmer
        """

class ExampleIndividual(BaseIndividual):
    element_class = _Chromosome

    def _fitness(self):
        # define the method to calculate the fitness
        x = self.decode()  # will call decode method of _Chromosome
        return evaluate(x)

If the chromosomes in an individual are different with each other, then subclass MixedIndividual, meanwhile, the property element_class should be assigned with a tuple of classes for each chromosome.

class MyIndividual(MixedIndividual):
    """
    Inherit the fitness from ExampleIndividual directly.
    It has 6 chromosomes, 5 are instances of _Chromosome, 1 is instance of FloatChromosome
    """
    element_class = (_Chromosome,)*5 + (FloatChromosome,)

It equivalent to MyIndividual=MixedIndividual[(_Chromosome,)*5 + (FloatChromosome,)]

Population

class MyPopulation(StandardPopulation):
    element_class = MyIndividual

It is equivalent to MyPopulation = StandardPopulation[MyIndividual].

Initialize randomly

random is a factory method!

Generate a population, with 50 individuals and each individual has 100 genes:

pop = MyPopulation.random(n_individuals=50, size=100)

When each individual contains 5 chromosomes, use

pop = MyPopulation.random(n_individuals=10, n_chromosomes=5, size=10)

However, we recommand to set default_size in the classes, then run MyPopulation.random()

class MyPopulation(StandardPopulation):
    element_class = MyIndividual // 5
    default_size = 10

# equiv. to

MyPopulation = StandardPopulation[MyIndividual//5]//10

In fact, random method of BasePopulation will call random method of BaseIndividual. If you want to generate an individual, then just execute MyIndividual.random(n_chromosomes=5, size=10), or set default_size, then execute MyIndividual.random().

Evolution

evolve method

Initialize a population with random method, then call evolve method.

pop = MyPopulation.random(n_individuals=50, size=100)
pop.evolve()
print(pop.solution)

set verbose=True to display the data for each generation.

evolve method mainly excute two methods:

• init: initial configuration of the algo.
• transition: do each step of the iteration.

History

Get the history of the evolution.

stat={'Mean Fitness':'mean_fitness', 'Best Fitness': lambda pop: pop.best_individual.fitness}
data = pop.history(stat=stat)  # use history instead of evolve

stat is a dict mapping keys to function, where string 'mean_fitness' means function lambda pop:pop.mean_fitness which gets the mean fitness of the individuals in pop. Since we have defined pop.best_individual.fitness as a property, stat could be redefined as {'Fitness': 'fitness', 'Best Fitness': 'max_fitness'}.

It requires ezstat, a easy statistical tool devoloped by the author.

performance

Use pop.perf() to check the performance, which calls evolve several times.

Example

Example 1

Description

select some of ti, ni, i=1,...,L, ti in {1,2,...,T}, ni in {1,2,...,N}
the sum of ni approx. 10, while ti dose not repeat

The opt. problem is

min abs(sum_i{ni}-10) + maximum of frequences in {ti}
where i is selected.

$$ \min_I |\sum_{i\in I} n_i -10| + t_m \ I\subset{1,\cdots,L} $$ where $t_m$ is the mode of ${t_i, i\in I}$

t = np.random.randint(1, 5, 100)
n = np.random.randint(1, 4, 100)

import collections


def max_repeat(x):
    # Maximum repetition
    c = collections.Counter(x)
    return np.max([b for a, b in c.items()])


class MyIndividual(makeBinaryIndividual()):

    def _fitness(self):
        x = abs(np.dot(n, self.chromosome)-10)
        y = max_repeat(ti for ti, c in zip(t, self) if c==1)
        return - x - y


class MyPopulation(StandardPopulation):
    element_class = MyIndividual

pop = MyPopulation.random(n_individuals=50, size=100)
pop.evolve()
print(pop.solution)  # or pop.best_individual.decode()

Note that there is only one chromosome in MonoIndividual, which could be got by self.chromosome.

In fact, the population could be the container of chromosomes. Therefore, we can rewrite the classes as follows in a more natural way.

class MyChromosome(BinaryChromosome):

    def _fitness(self):
        x = abs(np.dot(n, self)-10)
        y = max_repeat(ti for ti, c in zip(t, self) if c==1)
        return - x - y

class MyPopulation(StandardPopulation):
    element_class = MyChromosome

It is equiv. to

def _fitness(obj):
    x = abs(np.dot(n, obj)-10)
    y = max_repeat(ti for ti, c in zip(t, obj) if c==1)
    return - x - y

MyPopulation = StandardPopulation[BinaryChromosome].set_fitness(_fitness)

Example2: Knapsack Problem

One of the famous problem is the knapsack problem. It is a good example for GA.

We set history=True in evolve method for the example, that will record the main data of the whole evolution. It will return an object of pandas.DataFrame. The argument stat is a dict from a key to function/str(corresponding to a method) representing a mapping from a population to a number. these numbers of one generation will be stored in a row of the dataframe.

see # examples/example0

#!/usr/bin/env python3

from pyrimidine import binaryIndividual, StandardPopulation
from pyrimidine.benchmarks.optimization import *

# generate a knapsack problem randomly
evaluate = Knapsack.random(n=20)


class MyIndividual(binaryIndividual(size=20)):
    def _fitness(self):
        return evaluate(self)

class MyPopulation(StandardPopulation):
    element_class = MyIndividual
    default_size = 10


pop = MyPopulation.random()

stat={'Mean Fitness':'mean_fitness', 'Best Fitness':'max_fitness'}
data = pop.evolve(stat=stat, history=True) # an instance of `pandas.DataFrame`

# Visualization
import matplotlib.pyplot as plt
fig = plt.figure()
ax = fig.add_subplot(111)
data[['Mean Fitness', 'Best Fitness']].plot(ax=ax)
ax.set_xlabel('Generations')
ax.set_ylabel('Fitness')
plt.show()

Extension

pyrimidine is extremely extendable. It is easy to implement other iterative models or algorithms, such as simulation annealing(SA) and particle swarm optimization(PSO).

Currently, it is recommended to define subclasses based on IterativeModel as a mixin. (not mandatory)

In PSO, we regard a particle as an individual, and ParticleSwarm as a population. But in the following, we subclass it from IterativeModel

# pso.py
@basic_memory
class Particle(BaseIndividual):
    """A particle in PSO

    Extends BaseIndividual

    Variables:
        default_size {number} -- one individual represented by 2 chromosomes: position and velocity
        phantom {Particle} -- the current state of the particle moving in the solution space.
    """

    element_class = FloatChromosome
    default_size = 2

    # other methods

class ParticleSwarm(PopulationMixin):
    """Standard PSO
    
    Extends:
        PopulationMixin
    """

    element_class = Particle
    default_size = 20

    params = {'learning_factor': 2, 'acceleration_coefficient': 3,
    'inertia':0.75, 'n_best_particles':0.2, 'max_velocity':None}

    def init(self):
        for particle in self:
            particle.init()
        self.hall_of_fame = self.get_best_individuals(self.n_best_particles, copy=True)

    def update_hall_of_fame(self):
        hof_size = len(self.hall_of_fame)
        for ind in self:
            for k in range(hof_size):
                if self.hall_of_fame[-k-1].fitness < ind.fitness:
                    self.hall_of_fame.insert(hof_size-k, ind.copy())
                    self.hall_of_fame.pop(0)
                    break

    @property
    def best_fitness(self):
        if self.hall_of_fame:
            return max(map(attrgetter('fitness'), self.hall_of_fame))
        else:
            return super().best_fitness

    def transition(self, *args, **kwargs):
        """
        Transitation of the states of particles
        """
        self.move()
        self.backup()
        self.update_hall_of_fame()

    def backup(self):
        # overwrite the memory of the particle if its current state is better its memory
        for particle in self:
            particle.backup(check=True)

    def move(self):
        """Move the particles

        Define the moving rule of particles, according to the hall of fame and the best record
        """

        scale = random()
        eta = random()
        scale_fame = random()
        for particle in self:
            for fame in self.hall_of_fame:
                if particle.fitness < fame.fitness:
                    particle.update_vilocity_by_fame(fame, scale, scale_fame, 
                        self.inertia, self.learning_factor, self.acceleration_coefficient)
                    particle.position = particle.position + particle.velocity
                    break
        for particle in self.hall_of_fame:
            particle.update_vilocity(scale, self.inertia, self.learning_factor)
            particle.position = particle.position + particle.velocity

If you want to apply PSO, then you can define

class MyParticleSwarm(ParticleSwarm, BasePopulation):
    element_class = _Particle
    default_size = 20

pop = MyParticleSwarm.random()

Of course, it is not mandatory. It is allowed to inherit ParticleSwarm from for example HOFPopulation directly.