scipy.optimize.differential_evolution¶

scipy.optimize.
differential_evolution
(func, bounds, args=(), strategy='best1bin', maxiter=1000, popsize=15, tol=0.01, mutation=(0.5, 1), recombination=0.7, seed=None, callback=None, disp=False, polish=True, init='latinhypercube', atol=0, updating='immediate', workers=1)[source]¶ Finds the global minimum of a multivariate function.
Differential Evolution is stochastic in nature (does not use gradient methods) to find the minimium, and can search large areas of candidate space, but often requires larger numbers of function evaluations than conventional gradient based techniques.
The algorithm is due to Storn and Price [1].
Parameters:  func : callable
The objective function to be minimized. Must be in the form
f(x, *args)
, wherex
is the argument in the form of a 1D array andargs
is a tuple of any additional fixed parameters needed to completely specify the function. bounds : sequence
Bounds for variables.
(min, max)
pairs for each element inx
, defining the lower and upper bounds for the optimizing argument of func. It is required to havelen(bounds) == len(x)
.len(bounds)
is used to determine the number of parameters inx
. args : tuple, optional
Any additional fixed parameters needed to completely specify the objective function.
 strategy : str, optional
The differential evolution strategy to use. Should be one of:
 ‘best1bin’
 ‘best1exp’
 ‘rand1exp’
 ‘randtobest1exp’
 ‘currenttobest1exp’
 ‘best2exp’
 ‘rand2exp’
 ‘randtobest1bin’
 ‘currenttobest1bin’
 ‘best2bin’
 ‘rand2bin’
 ‘rand1bin’
The default is ‘best1bin’.
 maxiter : int, optional
The maximum number of generations over which the entire population is evolved. The maximum number of function evaluations (with no polishing) is:
(maxiter + 1) * popsize * len(x)
 popsize : int, optional
A multiplier for setting the total population size. The population has
popsize * len(x)
individuals (unless the initial population is supplied via the init keyword). tol : float, optional
Relative tolerance for convergence, the solving stops when
np.std(pop) <= atol + tol * np.abs(np.mean(population_energies))
, where and atol and tol are the absolute and relative tolerance respectively. mutation : float or tuple(float, float), optional
The mutation constant. In the literature this is also known as differential weight, being denoted by F. If specified as a float it should be in the range [0, 2]. If specified as a tuple
(min, max)
dithering is employed. Dithering randomly changes the mutation constant on a generation by generation basis. The mutation constant for that generation is taken fromU[min, max)
. Dithering can help speed convergence significantly. Increasing the mutation constant increases the search radius, but will slow down convergence. recombination : float, optional
The recombination constant, should be in the range [0, 1]. In the literature this is also known as the crossover probability, being denoted by CR. Increasing this value allows a larger number of mutants to progress into the next generation, but at the risk of population stability.
 seed : int or np.random.RandomState, optional
If seed is not specified the np.RandomState singleton is used. If seed is an int, a new np.random.RandomState instance is used, seeded with seed. If seed is already a np.random.RandomState instance, then that np.random.RandomState instance is used. Specify seed for repeatable minimizations.
 disp : bool, optional
Display status messages
 callback : callable, callback(xk, convergence=val), optional
A function to follow the progress of the minimization.
xk
is the current value ofx0
.val
represents the fractional value of the population convergence. Whenval
is greater than one the function halts. If callback returns True, then the minimization is halted (any polishing is still carried out). polish : bool, optional
If True (default), then
scipy.optimize.minimize
with the LBFGSB method is used to polish the best population member at the end, which can improve the minimization slightly. init : str or arraylike, optional
Specify which type of population initialization is performed. Should be one of:
 ‘latinhypercube’
 ‘random’
 array specifying the initial population. The array should have
shape
(M, len(x))
, where len(x) is the number of parameters. init is clipped to bounds before use.
The default is ‘latinhypercube’. Latin Hypercube sampling tries to maximize coverage of the available parameter space. ‘random’ initializes the population randomly  this has the drawback that clustering can occur, preventing the whole of parameter space being covered. Use of an array to specify a population subset could be used, for example, to create a tight bunch of initial guesses in an location where the solution is known to exist, thereby reducing time for convergence.
 atol : float, optional
Absolute tolerance for convergence, the solving stops when
np.std(pop) <= atol + tol * np.abs(np.mean(population_energies))
, where and atol and tol are the absolute and relative tolerance respectively. updating : {‘immediate’, ‘deferred’}, optional
If
'immediate'
, the best solution vector is continuously updated within a single generation [4]. This can lead to faster convergence as trial vectors can take advantage of continuous improvements in the best solution. With'deferred'
, the best solution vector is updated once per generation. Only'deferred'
is compatible with parallelization, and the workers keyword can override this option.New in version 1.2.0.
 workers : int or maplike callable, optional
If workers is an int the population is subdivided into workers sections and evaluated in parallel (uses
multiprocessing.Pool
). Supply 1 to use all available CPU cores. Alternatively supply a maplike callable, such as multiprocessing.Pool.map for evaluating the population in parallel. This evaluation is carried out asworkers(func, iterable)
. This option will override the updating keyword toupdating='deferred'
ifworkers != 1
. Requires that func be pickleable.New in version 1.2.0.
Returns:  res : OptimizeResult
The optimization result represented as a
OptimizeResult
object. Important attributes are:x
the solution array,success
a Boolean flag indicating if the optimizer exited successfully andmessage
which describes the cause of the termination. SeeOptimizeResult
for a description of other attributes. If polish was employed, and a lower minimum was obtained by the polishing, then OptimizeResult also contains thejac
attribute.
Notes
Differential evolution is a stochastic population based method that is useful for global optimization problems. At each pass through the population the algorithm mutates each candidate solution by mixing with other candidate solutions to create a trial candidate. There are several strategies [2] for creating trial candidates, which suit some problems more than others. The ‘best1bin’ strategy is a good starting point for many systems. In this strategy two members of the population are randomly chosen. Their difference is used to mutate the best member (the best in best1bin), \(b_0\), so far:
\[b' = b_0 + mutation * (population[rand0]  population[rand1])\]A trial vector is then constructed. Starting with a randomly chosen ‘i’th parameter the trial is sequentially filled (in modulo) with parameters from
b'
or the original candidate. The choice of whether to useb'
or the original candidate is made with a binomial distribution (the ‘bin’ in ‘best1bin’)  a random number in [0, 1) is generated. If this number is less than the recombination constant then the parameter is loaded fromb'
, otherwise it is loaded from the original candidate. The final parameter is always loaded fromb'
. Once the trial candidate is built its fitness is assessed. If the trial is better than the original candidate then it takes its place. If it is also better than the best overall candidate it also replaces that. To improve your chances of finding a global minimum use higher popsize values, with higher mutation and (dithering), but lower recombination values. This has the effect of widening the search radius, but slowing convergence. By default the best solution vector is updated continuously within a single iteration (updating='immediate'
). This is a modification [4] of the original differential evolution algorithm which can lead to faster convergence as trial vectors can immediately benefit from improved solutions. To use the original Storn and Price behaviour, updating the best solution once per iteration, setupdating='deferred'
.New in version 0.15.0.
References
[1] (1, 2) Storn, R and Price, K, Differential Evolution  a Simple and Efficient Heuristic for Global Optimization over Continuous Spaces, Journal of Global Optimization, 1997, 11, 341  359. [2] (1, 2) http://www1.icsi.berkeley.edu/~storn/code.html [3] http://en.wikipedia.org/wiki/Differential_evolution [4] (1, 2, 3) Wormington, M., Panaccione, C., Matney, K. M., Bowen, D. K.,  Characterization of structures from Xray scattering data using genetic algorithms, Phil. Trans. R. Soc. Lond. A, 1999, 357, 28272848 Examples
Let us consider the problem of minimizing the Rosenbrock function. This function is implemented in
rosen
inscipy.optimize
.>>> from scipy.optimize import rosen, differential_evolution >>> bounds = [(0,2), (0, 2), (0, 2), (0, 2), (0, 2)] >>> result = differential_evolution(rosen, bounds) >>> result.x, result.fun (array([1., 1., 1., 1., 1.]), 1.9216496320061384e19)
Now repeat, but with parallelization.
>>> bounds = [(0,2), (0, 2), (0, 2), (0, 2), (0, 2)] >>> result = differential_evolution(rosen, bounds, updating='deferred', ... workers=2) >>> result.x, result.fun (array([1., 1., 1., 1., 1.]), 1.9216496320061384e19)
Next find the minimum of the Ackley function (https://en.wikipedia.org/wiki/Test_functions_for_optimization).
>>> from scipy.optimize import differential_evolution >>> import numpy as np >>> def ackley(x): ... arg1 = 0.2 * np.sqrt(0.5 * (x[0] ** 2 + x[1] ** 2)) ... arg2 = 0.5 * (np.cos(2. * np.pi * x[0]) + np.cos(2. * np.pi * x[1])) ... return 20. * np.exp(arg1)  np.exp(arg2) + 20. + np.e >>> bounds = [(5, 5), (5, 5)] >>> result = differential_evolution(ackley, bounds) >>> result.x, result.fun (array([ 0., 0.]), 4.4408920985006262e16)