replace parallelize kwarg by reliance on cores setting

closes pymc-devs#3555

Author: michaelosthege

pymc3/sampling.py

@@ -452,7 +452,7 @@ def sample(draws=500, step=None, init='auto', n_init=200000, start=None, trace=N
             if has_population_samplers:
                 _log.info('Population sampling ({} chains)'.format(chains))
                 _print_step_hierarchy(step)
-                trace = _sample_population(**sample_args)
+                trace = _sample_population(**sample_args, parallelize=cores > 1)
             else:
                 _log.info('Sequential sampling ({} chains in 1 job)'.format(chains))
                 _print_step_hierarchy(step)
@@ -689,7 +689,7 @@ def __init__(self, steppers, parallelize):
         if parallelize:
             try:
                 # configure a child process for each stepper
-                _log.info('Attempting to parallelize chains.')
+                _log.info('Attempting to parallelize chains to all cores. You can turn this off with `pm.sample(cores=1)`.')
                 import multiprocessing
                 for c, stepper in enumerate(tqdm(steppers)):
                     slave_end, master_end = multiprocessing.Pipe()
@@ -714,7 +714,7 @@ def __init__(self, steppers, parallelize):
                 _log.debug('Error was: ', exec_info=True)
         else:
             _log.info('Chains are not parallelized. You can enable this by passing '
-                      'pm.sample(parallelize=True).')
+                      'pm.sample(cores=2).')
         return super().__init__()
 
     def __enter__(self):

pymc3/tests/test_step.py

@@ -920,7 +920,7 @@ def test_nonparallelized_chains_are_random(self):
             x = Normal("x", 0, 1)
             for stepper in TestPopulationSamplers.steppers:
                 step = stepper()
-                trace = sample(chains=4, draws=20, tune=0, step=DEMetropolis())
+                trace = sample(chains=4, cores=1, draws=20, tune=0, step=DEMetropolis())
                 samples = np.array(trace.get_values("x", combine=False))[:, 5]
 
                 assert len(set(samples)) == 4, "Parallelized {} " "chains are identical.".format(
@@ -933,7 +933,7 @@ def test_parallelized_chains_are_random(self):
             x = Normal("x", 0, 1)
             for stepper in TestPopulationSamplers.steppers:
                 step = stepper()
-                trace = sample(chains=4, draws=20, tune=0, step=DEMetropolis(), parallelize=True)
+                trace = sample(chains=4, cores=4, draws=20, tune=0, step=DEMetropolis())
                 samples = np.array(trace.get_values("x", combine=False))[:, 5]
 
                 assert len(set(samples)) == 4, "Parallelized {} " "chains are identical.".format(