diff --git a/pymc3/distributions/distribution.py b/pymc3/distributions/distribution.py
index 734722ae49..c8c895c188 100644
--- a/pymc3/distributions/distribution.py
+++ b/pymc3/distributions/distribution.py
@@ -54,6 +54,8 @@
     "vectorized_ppc", default=None
 )  # type: contextvars.ContextVar[Optional[Callable]]
 
+PLATFORM = sys.platform
+
 
 class _Unpickling:
     pass
@@ -510,17 +512,17 @@ def __init__(
         super().__init__(shape, dtype, testval, *args, **kwargs)
         self.logp = logp
         if type(self.logp) == types.MethodType:
-            if sys.platform != "linux":
+            if 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."
+                    "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."
+                    "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
diff --git a/pymc3/tests/test_parallel_sampling.py b/pymc3/tests/test_parallel_sampling.py
index f8063663e8..21715d6591 100644
--- a/pymc3/tests/test_parallel_sampling.py
+++ b/pymc3/tests/test_parallel_sampling.py
@@ -172,24 +172,23 @@ def func(x):
         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")
+        msg = "logp for DensityDist is a bound method, leading to RecursionError while serializing"
+        with pytest.raises(ValueError, match=msg):
+            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_syswarning(monkeypatch):
+    monkeypatch.setattr("pymc3.distributions.distribution.PLATFORM", "win32")
+    with pm.Model() as model:
+        mu = pm.Normal("mu", 0, 1)
+        normal_dist = pm.Normal.dist(mu, 1)
+        with pytest.warns(UserWarning, match="errors when sampling on platforms"):
+            obs = pm.DensityDist("density_dist", normal_dist.logp, observed=np.random.randn(100))
 
 
 def test_spawn_densitydist_mpctxwarning(monkeypatch):
@@ -198,6 +197,5 @@ def test_spawn_densitydist_mpctxwarning(monkeypatch):
     with pm.Model() as model:
         mu = pm.Normal("mu", 0, 1)
         normal_dist = pm.Normal.dist(mu, 1)
-        with pytest.warns(UserWarning) as w:
+        with pytest.warns(UserWarning, match="errors when sampling when multiprocessing"):
             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]