fix regression #4273 (#4297)

* informative warnings on bound method logp in DensityDist

* black

* run test on single core only to avoid dill error on windows/macos

* adding tests for DensityDist serialize recursion handling

* forgot a test

pymc3/distributions/distribution.py

@@ -12,11 +12,15 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 
+import multiprocessing
 import numbers
 import contextvars
 import dill
 import inspect
+import sys
+import types
 from typing import TYPE_CHECKING
+import warnings
 
 if TYPE_CHECKING:
     from typing import Optional, Callable
@@ -505,6 +509,19 @@ def __init__(
             dtype = theano.config.floatX
         super().__init__(shape, dtype, testval, *args, **kwargs)
         self.logp = logp
+        if type(self.logp) == types.MethodType:
+            if sys.platform != "linux":
+                warnings.warn(
+                    "You are passing a bound method as logp for DensityDist, this can lead to "
+                    + "errors when sampling on platforms other than Linux. Consider using a "
+                    + "plain function instead, or subclass Distribution."
+                )
+            elif type(multiprocessing.get_context()) != multiprocessing.context.ForkContext:
+                warnings.warn(
+                    "You are passing a bound method as logp for DensityDist, this can lead to "
+                    + "errors when sampling when multiprocessing cannot rely on forking. Consider using a "
+                    + "plain function instead, or subclass Distribution."
+                )
         self.rand = random
         self.wrap_random_with_dist_shape = wrap_random_with_dist_shape
         self.check_shape_in_random = check_shape_in_random
@@ -513,7 +530,15 @@ def __getstate__(self):
         # We use dill to serialize the logp function, as this is almost
         # always defined in the notebook and won't be pickled correctly.
         # Fix https://github.com/pymc-devs/pymc3/issues/3844
-        logp = dill.dumps(self.logp)
+        try:
+            logp = dill.dumps(self.logp)
+        except RecursionError as err:
+            if type(self.logp) == types.MethodType:
+                raise ValueError(
+                    "logp for DensityDist is a bound method, leading to RecursionError while serializing"
+                ) from err
+            else:
+                raise err
         vals = self.__dict__.copy()
         vals["logp"] = logp
         return vals

pymc3/tests/test_distributions_random.py

@@ -1171,7 +1171,7 @@ def test_density_dist_with_random_sampleable(self, shape):
                 shape=shape,
                 random=normal_dist.random,
             )
-            trace = pm.sample(100)
+            trace = pm.sample(100, cores=1)
 
         samples = 500
         size = 100
@@ -1194,7 +1194,7 @@ def test_density_dist_with_random_sampleable_failure(self, shape):
                 random=normal_dist.random,
                 wrap_random_with_dist_shape=False,
             )
-            trace = pm.sample(100)
+            trace = pm.sample(100, cores=1)
 
         samples = 500
         with pytest.raises(RuntimeError):
@@ -1217,7 +1217,7 @@ def test_density_dist_with_random_sampleable_hidden_error(self, shape):
                 wrap_random_with_dist_shape=False,
                 check_shape_in_random=False,
             )
-            trace = pm.sample(100)
+            trace = pm.sample(100, cores=1)
 
         samples = 500
         ppc = pm.sample_posterior_predictive(trace, samples=samples, model=model)
@@ -1240,7 +1240,7 @@ def test_density_dist_with_random_sampleable_handcrafted_success(self):
                 random=rvs,
                 wrap_random_with_dist_shape=False,
             )
-            trace = pm.sample(100)
+            trace = pm.sample(100, cores=1)
 
         samples = 500
         size = 100
@@ -1260,7 +1260,7 @@ def test_density_dist_with_random_sampleable_handcrafted_success_fast(self):
                 random=rvs,
                 wrap_random_with_dist_shape=False,
             )
-            trace = pm.sample(100)
+            trace = pm.sample(100, cores=1)
 
         samples = 500
         size = 100
@@ -1273,7 +1273,7 @@ def test_density_dist_without_random_not_sampleable(self):
             mu = pm.Normal("mu", 0, 1)
             normal_dist = pm.Normal.dist(mu, 1)
             pm.DensityDist("density_dist", normal_dist.logp, observed=np.random.randn(100))
-            trace = pm.sample(100)
+            trace = pm.sample(100, cores=1)
 
         samples = 500
         with pytest.raises(ValueError):

pymc3/tests/test_parallel_sampling.py

@@ -159,3 +159,45 @@ def test_iterator():
     with sampler:
         for draw in sampler:
             pass
+
+
+def test_spawn_densitydist_function():
+    with pm.Model() as model:
+        mu = pm.Normal("mu", 0, 1)
+
+        def func(x):
+            return -2 * (x ** 2).sum()
+
+        obs = pm.DensityDist("density_dist", func, observed=np.random.randn(100))
+        trace = pm.sample(draws=10, tune=10, step=pm.Metropolis(), cores=2, mp_ctx="spawn")
+
+
+@pytest.mark.xfail(raises=ValueError)
+def test_spawn_densitydist_bound_method():
+    with pm.Model() as model:
+        mu = pm.Normal("mu", 0, 1)
+        normal_dist = pm.Normal.dist(mu, 1)
+        obs = pm.DensityDist("density_dist", normal_dist.logp, observed=np.random.randn(100))
+        trace = pm.sample(draws=10, tune=10, step=pm.Metropolis(), cores=2, mp_ctx="spawn")
+
+
+# cannot test this properly: monkeypatching sys.platform messes up Theano
+# def test_spawn_densitydist_syswarning(monkeypatch):
+#     monkeypatch.setattr(sys, "platform", "win32")
+#     with pm.Model() as model:
+#         mu = pm.Normal('mu', 0, 1)
+#         normal_dist = pm.Normal.dist(mu, 1)
+#         with pytest.warns(UserWarning) as w:
+#             obs = pm.DensityDist('density_dist', normal_dist.logp, observed=np.random.randn(100))
+#         assert len(w) == 1 and "errors when sampling on platforms" in w[0].message.args[0]
+
+
+def test_spawn_densitydist_mpctxwarning(monkeypatch):
+    ctx = multiprocessing.get_context("spawn")
+    monkeypatch.setattr(multiprocessing, "get_context", lambda: ctx)
+    with pm.Model() as model:
+        mu = pm.Normal("mu", 0, 1)
+        normal_dist = pm.Normal.dist(mu, 1)
+        with pytest.warns(UserWarning) as w:
+            obs = pm.DensityDist("density_dist", normal_dist.logp, observed=np.random.randn(100))
+        assert len(w) == 1 and "errors when sampling when multiprocessing" in w[0].message.args[0]