Fix tests for extra divergence info

RELEASE-NOTES.md

@@ -1,6 +1,9 @@
 # Release Notes
 
 ## PyMC3 3.9.x (on deck)
+- Fix an error on Windows an Mac where error message from unpickling models did not show up in the notebook (see [#3991](https://github.com/pymc-devs/pymc3/pull/3991)).
+- Introduce an option argument `mp_ctx` to control how the processes for parallel sampling are started (see [#3991](https://github.com/pymc-devs/pymc3/pull/3991))
+
 ### Documentation
 - Notebook on [multilevel modeling](https://docs.pymc.io/notebooks/multilevel_modeling.html) has been rewritten to showcase ArviZ and xarray usage for inference result analysis (see [#3963](https://github.com/pymc-devs/pymc3/pull/3963))
 

pymc3/parallel_sampling.py

@@ -21,7 +21,6 @@
 from collections import namedtuple
 import traceback
 from pymc3.exceptions import SamplingError
-import errno
 
 import numpy as np
 from fastprogress.fastprogress import progress_bar
@@ -31,37 +30,6 @@
 logger = logging.getLogger("pymc3")
 
 
-def _get_broken_pipe_exception():
-    import sys
-
-    if sys.platform == "win32":
-        return RuntimeError(
-            "The communication pipe between the main process "
-            "and its spawned children is broken.\n"
-            "In Windows OS, this usually means that the child "
-            "process raised an exception while it was being "
-            "spawned, before it was setup to communicate to "
-            "the main process.\n"
-            "The exceptions raised by the child process while "
-            "spawning cannot be caught or handled from the "
-            "main process, and when running from an IPython or "
-            "jupyter notebook interactive kernel, the child's "
-            "exception and traceback appears to be lost.\n"
-            "A known way to see the child's error, and try to "
-            "fix or handle it, is to run the problematic code "
-            "as a batch script from a system's Command Prompt. "
-            "The child's exception will be printed to the "
-            "Command Promt's stderr, and it should be visible "
-            "above this error and traceback.\n"
-            "Note that if running a jupyter notebook that was "
-            "invoked from a Command Prompt, the child's "
-            "exception should have been printed to the Command "
-            "Prompt on which the notebook is running."
-        )
-    else:
-        return None
-
-
 class ParallelSamplingError(Exception):
     def __init__(self, message, chain, warnings=None):
         super().__init__(message)
@@ -133,18 +101,37 @@ def __init__(
         self._tune = tune
         self._pickle_backend = pickle_backend
 
+    def _unpickle_step_method(self):
+        unpickle_error = (
+            "The model could not be unpickled. This is required for sampling "
+            "with more than one core and multiprocessing context spawn "
+            "or forkserver."
+        )
+        if self._step_method_is_pickled:
+            if self._pickle_backend == 'pickle':
+                try:
+                    self._step_method = pickle.loads(self._step_method)
+                except Exception:
+                    raise ValueError(unpickle_error)
+            elif self._pickle_backend == 'dill':
+                try:
+                    import dill
+                except ImportError:
+                    raise ValueError(
+                        "dill must be installed for pickle_backend='dill'."
+                    )
+                try:
+                    self._step_method = dill.loads(self._step_method)
+                except Exception:
+                    raise ValueError(unpickle_error)
+            else:
+                raise ValueError("Unknown pickle backend")
+
     def run(self):
         try:
             # We do not create this in __init__, as pickling this
             # would destroy the shared memory.
-            if self._step_method_is_pickled:
-                if self._pickle_backend == 'pickle':
-                    self._step_method = pickle.loads(self._step_method)
-                elif self._pickle_backend == 'dill':
-                    import dill
-                    self._step_method = dill.loads(self._step_method)
-                else:
-                    raise ValueError("Unknown pickle backend")
+            self._unpickle_step_method()
             self._point = self._make_numpy_refs()
             self._start_loop()
         except KeyboardInterrupt:
@@ -289,7 +276,7 @@ def __init__(
 
         self._process = mp_ctx.Process(
             daemon=True,
-            name=name,
+            name=process_name,
             target=_run_process,
             args=(
                 process_name,
@@ -303,21 +290,10 @@ def __init__(
                 pickle_backend,
             )
         )
-        try:
-            self._process.start()
-            # Close the remote pipe, so that we get notified if the other
-            # end is closed.
-            remote_conn.close()
-        except IOError as e:
-            # Something may have gone wrong during the fork / spawn
-            if e.errno == errno.EPIPE:
-                exc = _get_broken_pipe_exception()
-                if exc is not None:
-                    # Sleep a little to give the child process time to flush
-                    # all its error message
-                    time.sleep(0.2)
-                    raise exc
-            raise
+        self._process.start()
+        # Close the remote pipe, so that we get notified if the other
+        # end is closed.
+        remote_conn.close()
 
     @property
     def shared_point_view(self):
@@ -451,7 +427,12 @@ def __init__(
             if pickle_backend == 'pickle':
                 step_method_pickled = pickle.dumps(step_method, protocol=-1)
             elif pickle_backend == 'dill':
-                import dill
+                try:
+                    import dill
+                except ImportError:
+                    raise ValueError(
+                        "dill must be installed for pickle_backend='dill'."
+                    )
                 step_method_pickled = dill.dumps(step_method, protocol=-1)
 
         self._samplers = [

pymc3/sampling.py

@@ -338,9 +338,13 @@ def sample(
         Defaults to `False`, but we'll switch to `True` in an upcoming release.
     idata_kwargs : dict, optional
         Keyword arguments for `arviz.from_pymc3`
-    mp_ctx : str
-        The name of a multiprocessing context. One of `fork`, `spawn` or `forkserver`.
-        See multiprocessing documentation for details.
+    mp_ctx : multiprocessing.context.BaseContent
+        A multiprocessing context for parallel sampling. See multiprocessing
+        documentation for details.
+    pickle_backend : str
+        One of `'pickle'` or `'dill'`. The library used to pickle models
+        in parallel sampling if the multiprocessing context is not of type
+        `fork`.
 
     Returns
     -------
@@ -508,8 +512,10 @@ def sample(
         "cores": cores,
         "callback": callback,
         "discard_tuned_samples": discard_tuned_samples,
-        "mp_ctx": mp_ctx,
+    }
+    parallel_args = {
         "pickle_backend": pickle_backend,
+        "mp_ctx": mp_ctx,
     }
 
     sample_args.update(kwargs)
@@ -527,7 +533,7 @@ def sample(
         _log.info("Multiprocess sampling ({} chains in {} jobs)".format(chains, cores))
         _print_step_hierarchy(step)
         try:
-            trace = _mp_sample(**sample_args)
+            trace = _mp_sample(**sample_args, **parallel_args)
         except pickle.PickleError:
             _log.warning("Could not pickle model, sampling singlethreaded.")
             _log.debug("Pickling error:", exec_info=True)

pymc3/tests/test_parallel_sampling.py

@@ -11,11 +11,41 @@
 #   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
+import multiprocessing
 
+import pytest
 import pymc3.parallel_sampling as ps
 import pymc3 as pm
 
 
+def test_context():
+    with pm.Model():
+        pm.Normal('x')
+        ctx = multiprocessing.get_context('spawn')
+        pm.sample(tune=2, draws=2, chains=2, cores=2, mp_ctx=ctx)
+
+
+class NoUnpickle:
+    def __getstate__(self):
+        return self.__dict__.copy()
+
+    def __setstate__(self, state):
+        raise AttributeError("This fails")
+
+
+def test_bad_unpickle():
+    with pm.Model() as model:
+        pm.Normal('x')
+
+    with model:
+        step = pm.NUTS()
+        step.no_unpickle = NoUnpickle()
+        with pytest.raises(Exception) as exc_info:
+            pm.sample(tune=2, draws=2, mp_ctx='spawn', step=step,
+                      cores=2, chains=2, compute_convergence_checks=False)
+        assert 'could not be unpickled' in str(exc_info.getrepr(style='short'))
+
+
 def test_abort():
     with pm.Model() as model:
         a = pm.Normal('a', shape=1)
@@ -25,8 +55,10 @@ def test_abort():
 
     step = pm.CompoundStep([step1, step2])
 
-    proc = ps.ProcessAdapter(10, 10, step, chain=3, seed=1,
-                             start={'a': 1., 'b_log__': 2.})
+    ctx = multiprocessing.get_context()
+    proc = ps.ProcessAdapter(10, 10, step, chain=3, seed=1, mp_ctx=ctx,
+                             start={'a': 1., 'b_log__': 2.},
+                             step_method_pickled=None, pickle_backend='pickle')
     proc.start()
     proc.write_next()
     proc.abort()
@@ -42,8 +74,10 @@ def test_explicit_sample():
 
     step = pm.CompoundStep([step1, step2])
 
-    proc = ps.ProcessAdapter(10, 10, step, chain=3, seed=1,
-                             start={'a': 1., 'b_log__': 2.})
+    ctx = multiprocessing.get_context()
+    proc = ps.ProcessAdapter(10, 10, step, chain=3, seed=1, mp_ctx=ctx,
+                             start={'a': 1., 'b_log__': 2.},
+                             step_method_pickled=None, pickle_backend='pickle')
     proc.start()
     while True:
         proc.write_next()