From 8092eed84662051aae168c36634152c21cd5fd81 Mon Sep 17 00:00:00 2001
From: Michael Osthege <m.osthege@fz-juelich.de>
Date: Mon, 9 Nov 2020 16:54:30 +0100
Subject: [PATCH 01/14] change tests to expect scalar results for (1,) shapes +
 by using more pytest.mark.parametrize, the overall number of overall tests
 increases, but the result should be easier to diagnose + an informative
 assert error message was added

---
 pymc3/tests/test_distributions_random.py | 96 +++++++-----------------
 1 file changed, 29 insertions(+), 67 deletions(-)

diff --git a/pymc3/tests/test_distributions_random.py b/pymc3/tests/test_distributions_random.py
index b15eb9b00e..fb3fb1548a 100644
--- a/pymc3/tests/test_distributions_random.py
+++ b/pymc3/tests/test_distributions_random.py
@@ -204,6 +204,7 @@ def setup_method(self, *args, **kwargs):
             self.model = pm.Model()
 
         def get_random_variable(self, shape, with_vector_params=False, name=None):
+            """ Creates a RandomVariable of the parametrized distribution. """
             if with_vector_params:
                 params = {
                     key: value * np.ones(self.shape, dtype=np.dtype(type(value)))
@@ -226,79 +227,40 @@ def get_random_variable(self, shape, with_vector_params=False, name=None):
 
         @staticmethod
         def sample_random_variable(random_variable, size):
+            """ Draws samples from a RandomVariable using its .random() method. """
             try:
                 return random_variable.random(size=size)
             except AttributeError:
                 return random_variable.distribution.random(size=size)
 
-        @pytest.mark.parametrize("size", [None, 5, (4, 5)], ids=str)
-        def test_scalar_parameter_shape(self, size):
-            rv = self.get_random_variable(None)
-            if size is None:
-                expected = (1,)
-            else:
-                expected = np.atleast_1d(size).tolist()
-            actual = np.atleast_1d(self.sample_random_variable(rv, size)).shape
-            assert tuple(expected) == actual
-
-        @pytest.mark.parametrize("size", [None, 5, (4, 5)], ids=str)
-        def test_scalar_shape(self, size):
-            shape = 10
+        @pytest.mark.parametrize("size", [None, (), 1, (1,), 5, (4, 5)], ids=str)
+        @pytest.mark.parametrize("shape", [None, ()], ids=str)
+        def test_scalar_distribution_shape(self, shape, size):
+            """ Draws samples of different [size] from a scalar [shape] RV. """
             rv = self.get_random_variable(shape)
-
-            if size is None:
-                expected = []
-            else:
-                expected = np.atleast_1d(size).tolist()
-            expected.append(shape)
-            actual = np.atleast_1d(self.sample_random_variable(rv, size)).shape
-            assert tuple(expected) == actual
-
-        @pytest.mark.parametrize("size", [None, 5, (4, 5)], ids=str)
-        def test_parameters_1d_shape(self, size):
-            rv = self.get_random_variable(self.shape, with_vector_params=True)
-            if size is None:
-                expected = []
-            else:
-                expected = np.atleast_1d(size).tolist()
-            expected.append(self.shape)
-            actual = self.sample_random_variable(rv, size).shape
-            assert tuple(expected) == actual
-
-        @pytest.mark.parametrize("size", [None, 5, (4, 5)], ids=str)
-        def test_broadcast_shape(self, size):
-            broadcast_shape = (2 * self.shape, self.shape)
-            rv = self.get_random_variable(broadcast_shape, with_vector_params=True)
-            if size is None:
-                expected = []
-            else:
-                expected = np.atleast_1d(size).tolist()
-            expected.extend(broadcast_shape)
-            actual = np.atleast_1d(self.sample_random_variable(rv, size)).shape
-            assert tuple(expected) == actual
-
-        @pytest.mark.parametrize(
-            "shape", [(), (1,), (1, 1), (1, 2), (10, 10, 1), (10, 10, 2)], ids=str
-        )
-        def test_different_shapes_and_sample_sizes(self, shape):
-            prefix = self.distribution.__name__
-
-            rv = self.get_random_variable(shape, name=f"{prefix}_{shape}")
-            for size in (None, 1, 5, (4, 5)):
-                if size is None:
-                    s = []
-                else:
-                    try:
-                        s = list(size)
-                    except TypeError:
-                        s = [size]
-                    if s == [1]:
-                        s = []
-                if shape not in ((), (1,)):
-                    s.extend(shape)
-                e = tuple(s)
-                a = self.sample_random_variable(rv, size).shape
-                assert e == a
+            expected = () if size in {None, ()} else tuple(np.atleast_1d(size))
+            actual = np.shape(self.sample_random_variable(rv, size))
+            assert expected == actual, f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
+
+        @pytest.mark.parametrize("size", [None, ()], ids=str)
+        @pytest.mark.parametrize("shape", [None, (), (1,), (1, 1), (1, 2), (10, 11, 1), (9, 10, 2)], ids=str)
+        def test_scalar_sample_shape(self, shape, size):
+            """ Draws samples of scalar [size] from a [shape] RV. """
+            rv = self.get_random_variable(shape)
+            expected = () if shape in {None, ()} else tuple(np.atleast_1d(shape))
+            actual = np.shape(self.sample_random_variable(rv, size))
+            assert expected == actual, f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
+
+        @pytest.mark.parametrize("size", [None, 3, (4, 5)], ids=str)
+        @pytest.mark.parametrize("shape", [None, 1, (10, 11, 1)], ids=str)
+        def test_vector_params(self, shape, size):
+            shape = self.shape
+            rv = self.get_random_variable(shape, with_vector_params=True)
+            exp_shape = () if shape in {None, ()} else tuple(np.atleast_1d(shape))
+            exp_size = () if size in {None, ()} else tuple(np.atleast_1d(size))
+            expected = exp_size + exp_shape
+            actual = np.shape(self.sample_random_variable(rv, size))
+            assert expected == actual, f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
 
 
 class TestGaussianRandomWalk(BaseTestCases.BaseTestCase):

From fbb6fe2a365bd4c0dfd25646ccd3f3422ae1b971 Mon Sep 17 00:00:00 2001
From: Michael Osthege <m.osthege@fz-juelich.de>
Date: Mon, 9 Nov 2020 17:00:37 +0100
Subject: [PATCH 02/14] only treat () as scalar shapes closes #4206

---
 pymc3/distributions/distribution.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/pymc3/distributions/distribution.py b/pymc3/distributions/distribution.py
index a32031ebb8..2799049ea3 100644
--- a/pymc3/distributions/distribution.py
+++ b/pymc3/distributions/distribution.py
@@ -943,11 +943,9 @@ def _draw_value(param, point=None, givens=None, size=None):
 
 
 def _is_one_d(dist_shape):
-    if hasattr(dist_shape, "dshape") and dist_shape.dshape in ((), (0,), (1,)):
+    if hasattr(dist_shape, "dshape") and dist_shape.dshape in { (), }:
         return True
-    elif hasattr(dist_shape, "shape") and dist_shape.shape in ((), (0,), (1,)):
-        return True
-    elif to_tuple(dist_shape) == ():
+    elif hasattr(dist_shape, "shape") and dist_shape.shape in { (), }:
         return True
     return False
 
@@ -1069,6 +1067,7 @@ def generate_samples(generator, *args, **kwargs):
             len(samples.shape) > len(dist_shape)
             and samples.shape[-len(dist_shape) :] == dist_shape[-len(dist_shape) :]
         ):
+            raise ValueError(f"This SHOULD be unreachable code. DON'T MERGE UNTIL THIS ENTIRE BLOCK WAS REMOVED. {samples.shape}, {size_tup}")
             samples = samples.reshape(samples.shape[1:])
 
     if (
@@ -1077,5 +1076,6 @@ def generate_samples(generator, *args, **kwargs):
         and samples.shape[-1] == 1
         and (samples.shape != size_tup or size_tup == tuple() or size_tup == (1,))
     ):
+        raise ValueError(f"This SHOULD be unreachable code. DON'T MERGE UNTIL THIS ENTIRE BLOCK WAS REMOVED. {samples.shape}, {size_tup}")
         samples = samples.reshape(samples.shape[:-1])
     return np.asarray(samples)

From 058920345365e145d283a56db4935e33566f62d6 Mon Sep 17 00:00:00 2001
From: Michael Osthege <m.osthege@fz-juelich.de>
Date: Mon, 9 Nov 2020 17:09:50 +0100
Subject: [PATCH 03/14] fixes shape tests with new scalar behavior

---
 pymc3/distributions/discrete.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pymc3/distributions/discrete.py b/pymc3/distributions/discrete.py
index fd2d399618..53babb28dc 100644
--- a/pymc3/distributions/discrete.py
+++ b/pymc3/distributions/discrete.py
@@ -972,7 +972,7 @@ def random(self, point=None, size=None):
         return generate_samples(
             random_choice,
             p=p,
-            broadcast_shape=p.shape[:-1] or (1,),
+            broadcast_shape=p.shape[:-1],
             dist_shape=self.shape,
             size=size,
         )

From d89f87928ec304f186aef45c0b902c42a262f908 Mon Sep 17 00:00:00 2001
From: Michael Osthege <m.osthege@fz-juelich.de>
Date: Mon, 9 Nov 2020 17:50:42 +0100
Subject: [PATCH 04/14] fix BetaBinomial new scalar shape handling

---
 pymc3/distributions/discrete.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pymc3/distributions/discrete.py b/pymc3/distributions/discrete.py
index 53babb28dc..4b183617d1 100644
--- a/pymc3/distributions/discrete.py
+++ b/pymc3/distributions/discrete.py
@@ -200,7 +200,7 @@ def __init__(self, alpha, beta, n, *args, **kwargs):
         self.mode = tt.cast(tround(alpha / (alpha + beta)), "int8")
 
     def _random(self, alpha, beta, n, size=None):
-        size = size or 1
+        size = size or ()
         p = stats.beta.rvs(a=alpha, b=beta, size=size).flatten()
         # Sometimes scipy.beta returns nan. Ugh.
         while np.any(np.isnan(p)):

From 08caafa4dcbaff6762c7139f8bd1d541734844f9 Mon Sep 17 00:00:00 2001
From: Michael Osthege <m.osthege@fz-juelich.de>
Date: Mon, 9 Nov 2020 18:09:19 +0100
Subject: [PATCH 05/14] remove sample reshaping

---
 pymc3/distributions/distribution.py | 19 +------------------
 1 file changed, 1 insertion(+), 18 deletions(-)

diff --git a/pymc3/distributions/distribution.py b/pymc3/distributions/distribution.py
index 2799049ea3..655528cd58 100644
--- a/pymc3/distributions/distribution.py
+++ b/pymc3/distributions/distribution.py
@@ -983,6 +983,7 @@ def generate_samples(generator, *args, **kwargs):
     Any remaining args and kwargs are passed on to the generator function.
     """
     dist_shape = kwargs.pop("dist_shape", ())
+    # TODO: the following variable is no longer used !!
     one_d = _is_one_d(dist_shape)
     size = kwargs.pop("size", None)
     broadcast_shape = kwargs.pop("broadcast_shape", None)
@@ -1059,23 +1060,5 @@ def generate_samples(generator, *args, **kwargs):
         samples = generator(size=dist_bcast_shape, *args, **kwargs)
     else:
         samples = generator(size=size_tup + dist_bcast_shape, *args, **kwargs)
-    samples = np.asarray(samples)
 
-    # reshape samples here
-    if samples.ndim > 0 and samples.shape[0] == 1 and size_tup == (1,):
-        if (
-            len(samples.shape) > len(dist_shape)
-            and samples.shape[-len(dist_shape) :] == dist_shape[-len(dist_shape) :]
-        ):
-            raise ValueError(f"This SHOULD be unreachable code. DON'T MERGE UNTIL THIS ENTIRE BLOCK WAS REMOVED. {samples.shape}, {size_tup}")
-            samples = samples.reshape(samples.shape[1:])
-
-    if (
-        one_d
-        and samples.ndim > 0
-        and samples.shape[-1] == 1
-        and (samples.shape != size_tup or size_tup == tuple() or size_tup == (1,))
-    ):
-        raise ValueError(f"This SHOULD be unreachable code. DON'T MERGE UNTIL THIS ENTIRE BLOCK WAS REMOVED. {samples.shape}, {size_tup}")
-        samples = samples.reshape(samples.shape[:-1])
     return np.asarray(samples)

From 21de4465151944192457c17c2578a1237c13fa47 Mon Sep 17 00:00:00 2001
From: Michael Osthege <m.osthege@fz-juelich.de>
Date: Mon, 9 Nov 2020 18:10:46 +0100
Subject: [PATCH 06/14] adaptions for timeseries distributions

---
 pymc3/tests/test_distributions_random.py | 50 +++++++++++++++---------
 1 file changed, 32 insertions(+), 18 deletions(-)

diff --git a/pymc3/tests/test_distributions_random.py b/pymc3/tests/test_distributions_random.py
index fb3fb1548a..007834704b 100644
--- a/pymc3/tests/test_distributions_random.py
+++ b/pymc3/tests/test_distributions_random.py
@@ -198,6 +198,12 @@ def test_blocking_context(self):
 class BaseTestCases:
     class BaseTestCase(SeededTest):
         shape = 5
+        # the following are the default values of the distribution that take effect
+        # when the parametrized shape/size in the test case is None.
+        # For every distribution that defaults to non-scalar shapes they must be
+        # specified by the inheriting Test class. example: TestGaussianRandomWalk
+        default_shape = ()
+        default_size = ()
 
         def setup_method(self, *args, **kwargs):
             super().setup_method(*args, **kwargs)
@@ -215,30 +221,39 @@ def get_random_variable(self, shape, with_vector_params=False, name=None):
             if name is None:
                 name = self.distribution.__name__
             with self.model:
-                if shape is None:
-                    return self.distribution(name, transform=None, **params)
-                else:
-                    try:
+                try:
+                    if shape is None:
+                        # in the test case parametrization "None" means "no specified (default)"
+                        return self.distribution(name, transform=None, **params)
+                    else:
                         return self.distribution(name, shape=shape, transform=None, **params)
-                    except TypeError:
-                        if np.sum(np.atleast_1d(shape)) == 0:
-                            pytest.skip("Timeseries must have positive shape")
-                        raise
+                except TypeError:
+                    if np.sum(np.atleast_1d(shape)) == 0:
+                        pytest.skip("Timeseries must have positive shape")
+                    raise
 
         @staticmethod
         def sample_random_variable(random_variable, size):
             """ Draws samples from a RandomVariable using its .random() method. """
             try:
-                return random_variable.random(size=size)
+                if size is None:
+                    return random_variable.random()
+                else:
+                    return random_variable.random(size=size)
             except AttributeError:
-                return random_variable.distribution.random(size=size)
+                if size is None:
+                    return random_variable.distribution.random()
+                else:
+                    return random_variable.distribution.random(size=size)
 
         @pytest.mark.parametrize("size", [None, (), 1, (1,), 5, (4, 5)], ids=str)
         @pytest.mark.parametrize("shape", [None, ()], ids=str)
         def test_scalar_distribution_shape(self, shape, size):
             """ Draws samples of different [size] from a scalar [shape] RV. """
             rv = self.get_random_variable(shape)
-            expected = () if size in {None, ()} else tuple(np.atleast_1d(size))
+            exp_shape = self.default_shape if shape is None else tuple(np.atleast_1d(shape))
+            exp_size = self.default_size if size is None else tuple(np.atleast_1d(size))
+            expected = exp_size + exp_shape
             actual = np.shape(self.sample_random_variable(rv, size))
             assert expected == actual, f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
 
@@ -247,7 +262,9 @@ def test_scalar_distribution_shape(self, shape, size):
         def test_scalar_sample_shape(self, shape, size):
             """ Draws samples of scalar [size] from a [shape] RV. """
             rv = self.get_random_variable(shape)
-            expected = () if shape in {None, ()} else tuple(np.atleast_1d(shape))
+            exp_shape = self.default_shape if shape is None else tuple(np.atleast_1d(shape))
+            exp_size = self.default_size if size is None else tuple(np.atleast_1d(size))
+            expected = exp_size + exp_shape
             actual = np.shape(self.sample_random_variable(rv, size))
             assert expected == actual, f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
 
@@ -256,8 +273,8 @@ def test_scalar_sample_shape(self, shape, size):
         def test_vector_params(self, shape, size):
             shape = self.shape
             rv = self.get_random_variable(shape, with_vector_params=True)
-            exp_shape = () if shape in {None, ()} else tuple(np.atleast_1d(shape))
-            exp_size = () if size in {None, ()} else tuple(np.atleast_1d(size))
+            exp_shape = self.default_shape if shape is None else tuple(np.atleast_1d(shape))
+            exp_size = self.default_size if size is None else tuple(np.atleast_1d(size))
             expected = exp_size + exp_shape
             actual = np.shape(self.sample_random_variable(rv, size))
             assert expected == actual, f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
@@ -266,10 +283,7 @@ def test_vector_params(self, shape, size):
 class TestGaussianRandomWalk(BaseTestCases.BaseTestCase):
     distribution = pm.GaussianRandomWalk
     params = {"mu": 1.0, "sigma": 1.0}
-
-    @pytest.mark.xfail(reason="Supporting this makes a nasty API")
-    def test_broadcast_shape(self):
-        super().test_broadcast_shape()
+    default_shape = (1,)
 
 
 class TestNormal(BaseTestCases.BaseTestCase):

From 309507170967de12749c3560da8631d4dd966d44 Mon Sep 17 00:00:00 2001
From: Michael Osthege <m.osthege@fz-juelich.de>
Date: Tue, 10 Nov 2020 09:51:54 +0100
Subject: [PATCH 07/14] take out old (1,) shape handling

---
 pymc3/distributions/posterior_predictive.py | 2 --
 pymc3/tests/test_shape_handling.py          | 3 ---
 2 files changed, 5 deletions(-)

diff --git a/pymc3/distributions/posterior_predictive.py b/pymc3/distributions/posterior_predictive.py
index 7a4ab43d30..c5d621318b 100644
--- a/pymc3/distributions/posterior_predictive.py
+++ b/pymc3/distributions/posterior_predictive.py
@@ -558,8 +558,6 @@ def random_sample(
             shape: Tuple[int, ...],
         ) -> np.ndarray:
             val = meth(point=point, size=size)
-            if size == 1:
-                val = np.expand_dims(val, axis=0)
             try:
                 assert val.shape == (size,) + shape, (
                     "Sampling from random of %s yields wrong shape" % param
diff --git a/pymc3/tests/test_shape_handling.py b/pymc3/tests/test_shape_handling.py
index 50ed4a24fd..9708aa9fb3 100644
--- a/pymc3/tests/test_shape_handling.py
+++ b/pymc3/tests/test_shape_handling.py
@@ -211,9 +211,6 @@ def test_broadcast_dist_samples_to(self, samples_to_broadcast_to):
 def test_sample_generate_values(fixture_model, fixture_sizes):
     model, RVs = fixture_model
     size = to_tuple(fixture_sizes)
-    if size == (1,):
-        # Single draws are interpreted as scalars for backwards compatibility
-        size = tuple()
     with model:
         prior = pm.sample_prior_predictive(samples=fixture_sizes)
         for rv in RVs:

From 2e38d89ab9fe882211081128c17b6c8ef117c293 Mon Sep 17 00:00:00 2001
From: Michael Osthege <m.osthege@fz-juelich.de>
Date: Wed, 11 Nov 2020 13:01:08 +0100
Subject: [PATCH 08/14] black formatting

---
 pymc3/distributions/distribution.py      |  4 ++--
 pymc3/tests/test_distributions_random.py | 16 ++++++++++++----
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/pymc3/distributions/distribution.py b/pymc3/distributions/distribution.py
index 655528cd58..b419fe9b8b 100644
--- a/pymc3/distributions/distribution.py
+++ b/pymc3/distributions/distribution.py
@@ -943,9 +943,9 @@ def _draw_value(param, point=None, givens=None, size=None):
 
 
 def _is_one_d(dist_shape):
-    if hasattr(dist_shape, "dshape") and dist_shape.dshape in { (), }:
+    if hasattr(dist_shape, "dshape") and dist_shape.dshape in {()}:
         return True
-    elif hasattr(dist_shape, "shape") and dist_shape.shape in { (), }:
+    elif hasattr(dist_shape, "shape") and dist_shape.shape in {()}:
         return True
     return False
 
diff --git a/pymc3/tests/test_distributions_random.py b/pymc3/tests/test_distributions_random.py
index 007834704b..d46fc317f1 100644
--- a/pymc3/tests/test_distributions_random.py
+++ b/pymc3/tests/test_distributions_random.py
@@ -255,10 +255,14 @@ def test_scalar_distribution_shape(self, shape, size):
             exp_size = self.default_size if size is None else tuple(np.atleast_1d(size))
             expected = exp_size + exp_shape
             actual = np.shape(self.sample_random_variable(rv, size))
-            assert expected == actual, f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
+            assert (
+                expected == actual
+            ), f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
 
         @pytest.mark.parametrize("size", [None, ()], ids=str)
-        @pytest.mark.parametrize("shape", [None, (), (1,), (1, 1), (1, 2), (10, 11, 1), (9, 10, 2)], ids=str)
+        @pytest.mark.parametrize(
+            "shape", [None, (), (1,), (1, 1), (1, 2), (10, 11, 1), (9, 10, 2)], ids=str
+        )
         def test_scalar_sample_shape(self, shape, size):
             """ Draws samples of scalar [size] from a [shape] RV. """
             rv = self.get_random_variable(shape)
@@ -266,7 +270,9 @@ def test_scalar_sample_shape(self, shape, size):
             exp_size = self.default_size if size is None else tuple(np.atleast_1d(size))
             expected = exp_size + exp_shape
             actual = np.shape(self.sample_random_variable(rv, size))
-            assert expected == actual, f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
+            assert (
+                expected == actual
+            ), f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
 
         @pytest.mark.parametrize("size", [None, 3, (4, 5)], ids=str)
         @pytest.mark.parametrize("shape", [None, 1, (10, 11, 1)], ids=str)
@@ -277,7 +283,9 @@ def test_vector_params(self, shape, size):
             exp_size = self.default_size if size is None else tuple(np.atleast_1d(size))
             expected = exp_size + exp_shape
             actual = np.shape(self.sample_random_variable(rv, size))
-            assert expected == actual, f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
+            assert (
+                expected == actual
+            ), f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
 
 
 class TestGaussianRandomWalk(BaseTestCases.BaseTestCase):

From eb0fcf86e51d61eda0642da55cc6515a752abc19 Mon Sep 17 00:00:00 2001
From: Michael Osthege <michael.osthege@outlook.com>
Date: Sun, 13 Dec 2020 23:55:57 +0100
Subject: [PATCH 09/14] raise ValueError when Distribution is initialized with
 a 0-length dimension

---
 pymc3/distributions/distribution.py      | 6 ++++++
 pymc3/tests/test_distributions_random.py | 8 ++++++++
 2 files changed, 14 insertions(+)

diff --git a/pymc3/distributions/distribution.py b/pymc3/distributions/distribution.py
index 23749f1c47..42125777f9 100644
--- a/pymc3/distributions/distribution.py
+++ b/pymc3/distributions/distribution.py
@@ -107,6 +107,12 @@ def __new__(cls, name, *args, **kwargs):
                 dims = (dims,)
             shape = model.shape_from_dims(dims)
 
+        # failsafe against 0-shapes
+        if shape is not None and any(np.atleast_1d(shape) <= 0):
+            raise ValueError(
+                f"Distribution initialized with invalid shape {shape}. This is not allowed."
+            )
+
         # Some distributions do not accept shape=None
         if has_shape or shape is not None:
             dist = cls.dist(*args, **kwargs, shape=shape)
diff --git a/pymc3/tests/test_distributions_random.py b/pymc3/tests/test_distributions_random.py
index 186471bfb3..34f07ff3a8 100644
--- a/pymc3/tests/test_distributions_random.py
+++ b/pymc3/tests/test_distributions_random.py
@@ -264,6 +264,9 @@ def test_scalar_distribution_shape(self, shape, size):
             assert (
                 expected == actual
             ), f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
+            # check that negative size raises an error
+            with pytest.raises(ValueError, match="not allowed"):
+                rv.random(size=-2)
 
         @pytest.mark.parametrize("size", [None, ()], ids=str)
         @pytest.mark.parametrize(
@@ -293,6 +296,11 @@ def test_vector_params(self, shape, size):
                 expected == actual
             ), f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
 
+        @pytest.mark.parametrize("shape", [-2, 0, (0,), (2, 0), (5, 0, 3)])
+        def test_shape_error_on_zero_shape_rv(self, shape):
+            with pytest.raises(ValueError, match="not allowed"):
+                self.get_random_variable(shape)
+
 
 class TestGaussianRandomWalk(BaseTestCases.BaseTestCase):
     distribution = pm.GaussianRandomWalk

From 7eb78a716804de38b1411f8c42349ff96b3b4068 Mon Sep 17 00:00:00 2001
From: Michael Osthege <michael.osthege@outlook.com>
Date: Mon, 14 Dec 2020 11:47:11 +0100
Subject: [PATCH 10/14] Don't match ValueError message because some
 distributions use different texts

---
 pymc3/tests/test_distributions_random.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/pymc3/tests/test_distributions_random.py b/pymc3/tests/test_distributions_random.py
index 34f07ff3a8..7baf3e3999 100644
--- a/pymc3/tests/test_distributions_random.py
+++ b/pymc3/tests/test_distributions_random.py
@@ -265,8 +265,10 @@ def test_scalar_distribution_shape(self, shape, size):
                 expected == actual
             ), f"Sample size {size} from {shape}-shaped RV had shape {actual}. Expected: {expected}"
             # check that negative size raises an error
-            with pytest.raises(ValueError, match="not allowed"):
-                rv.random(size=-2)
+            with pytest.raises(ValueError):
+                self.sample_random_variable(rv, size=-2)
+            with pytest.raises(ValueError):
+                self.sample_random_variable(rv, size=(3, -2))
 
         @pytest.mark.parametrize("size", [None, ()], ids=str)
         @pytest.mark.parametrize(

From 3ff49594d89c303eff947459c45ba1590b9e66b5 Mon Sep 17 00:00:00 2001
From: Michael Osthege <michael.osthege@outlook.com>
Date: Mon, 14 Dec 2020 16:12:14 +0100
Subject: [PATCH 11/14] Update releasenotes and apply review feedback

---
 RELEASE-NOTES.md                    | 11 ++++++++++-
 pymc3/distributions/distribution.py | 10 ----------
 2 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/RELEASE-NOTES.md b/RELEASE-NOTES.md
index 2a7b8f5796..106e4c418e 100644
--- a/RELEASE-NOTES.md
+++ b/RELEASE-NOTES.md
@@ -1,11 +1,20 @@
 # Release Notes
 
-## PyMC3 3.10.1 (on deck)
+## PyMC3 3.11.0 (on deck)
+This release breaks some APIs w.r.t. `3.10.0`.
+It also brings some dreadfully awaited fixes, so be sure to go through the changes below.
+(Or latest when you run into problems.)
 
 ### Maintenance
+- Changed shape behavior: __No longer collapse length 1 vector shape into scalars.__ (see [#4206](https://github.com/pymc-devs/pymc3/issue/4206) and [#4214](https://github.com/pymc-devs/pymc3/pull/4214))
+  - __Applies to random variables and also the `.random(size=...)` kwarg!__
+  - To create scalar variables you must now use `shape=None` or `shape=()`.
+  - __`shape=(1,)` and `shape=1` now become vectors.__ Previously they were collapsed into scalars
+  - 0-length dimensions are now ruled illegal for random variables and raise a `ValueError`.
 - Fixed bug whereby partial traces returns after keyboard interrupt during parallel sampling had fewer draws than would've been available [#4318](https://github.com/pymc-devs/pymc3/pull/4318)
 - Make `sample_shape` same across all contexts in `draw_values` (see [#4305](https://github.com/pymc-devs/pymc3/pull/4305)).
 - Removed `theanof.set_theano_config` because it illegally touched Theano's privates (see [#4329](https://github.com/pymc-devs/pymc3/pull/4329)).
+- In `sample_prior_predictive` the `vars` kwarg was removed in favor of `var_names` (see [#4327](https://github.com/pymc-devs/pymc3/pull/4327))
 
 
 ## PyMC3 3.10.0 (7 December 2020)
diff --git a/pymc3/distributions/distribution.py b/pymc3/distributions/distribution.py
index 42125777f9..2ab840c536 100644
--- a/pymc3/distributions/distribution.py
+++ b/pymc3/distributions/distribution.py
@@ -1007,14 +1007,6 @@ def _draw_value(param, point=None, givens=None, size=None):
     raise ValueError("Unexpected type in draw_value: %s" % type(param))
 
 
-def _is_one_d(dist_shape):
-    if hasattr(dist_shape, "dshape") and dist_shape.dshape in {()}:
-        return True
-    elif hasattr(dist_shape, "shape") and dist_shape.shape in {()}:
-        return True
-    return False
-
-
 def generate_samples(generator, *args, **kwargs):
     """Generate samples from the distribution of a random variable.
 
@@ -1048,8 +1040,6 @@ def generate_samples(generator, *args, **kwargs):
     Any remaining args and kwargs are passed on to the generator function.
     """
     dist_shape = kwargs.pop("dist_shape", ())
-    # TODO: the following variable is no longer used !!
-    one_d = _is_one_d(dist_shape)
     size = kwargs.pop("size", None)
     broadcast_shape = kwargs.pop("broadcast_shape", None)
     not_broadcast_kwargs = kwargs.pop("not_broadcast_kwargs", None)

From 1652e4b7ecb7b3e9ad165ad63385da5dabc5b546 Mon Sep 17 00:00:00 2001
From: Sayam Kumar <sayamkumar753@yahoo.in>
Date: Fri, 18 Dec 2020 13:30:02 +0530
Subject: [PATCH 12/14] Removed xfail tests while testing mvnormal random

---
 pymc3/tests/test_distributions_random.py | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/pymc3/tests/test_distributions_random.py b/pymc3/tests/test_distributions_random.py
index 7baf3e3999..35e8710df1 100644
--- a/pymc3/tests/test_distributions_random.py
+++ b/pymc3/tests/test_distributions_random.py
@@ -1571,7 +1571,7 @@ def test_Triangular(
         assert prior["target"].shape == (prior_samples,) + shape
 
 
-def generate_shapes(include_params=False, xfail=False):
+def generate_shapes(include_params=False):
     # fmt: off
     mudim_as_event = [
         [None, 1, 3, 10, (10, 3), 100],
@@ -1590,20 +1590,13 @@ def generate_shapes(include_params=False, xfail=False):
         del mudim_as_event[-1]
         del mudim_as_dist[-1]
     data = itertools.chain(itertools.product(*mudim_as_event), itertools.product(*mudim_as_dist))
-    if xfail:
-        data = list(data)
-        for index in range(len(data)):
-            if data[index][0] in (None, 1):
-                data[index] = pytest.param(
-                    *data[index], marks=pytest.mark.xfail(reason="wait for PR #4214")
-                )
     return data
 
 
 class TestMvNormal(SeededTest):
     @pytest.mark.parametrize(
         ["sample_shape", "dist_shape", "mu_shape", "param"],
-        generate_shapes(include_params=True, xfail=False),
+        generate_shapes(include_params=True),
         ids=str,
     )
     def test_with_np_arrays(self, sample_shape, dist_shape, mu_shape, param):
@@ -1613,7 +1606,7 @@ def test_with_np_arrays(self, sample_shape, dist_shape, mu_shape, param):
 
     @pytest.mark.parametrize(
         ["sample_shape", "dist_shape", "mu_shape"],
-        generate_shapes(include_params=False, xfail=True),
+        generate_shapes(include_params=False),
         ids=str,
     )
     def test_with_chol_rv(self, sample_shape, dist_shape, mu_shape):
@@ -1630,7 +1623,7 @@ def test_with_chol_rv(self, sample_shape, dist_shape, mu_shape):
 
     @pytest.mark.parametrize(
         ["sample_shape", "dist_shape", "mu_shape"],
-        generate_shapes(include_params=False, xfail=True),
+        generate_shapes(include_params=False),
         ids=str,
     )
     def test_with_cov_rv(self, sample_shape, dist_shape, mu_shape):

From 3e52058bbf814e3496479829f838e439d85f8b6e Mon Sep 17 00:00:00 2001
From: Michael Osthege <michael.osthege@outlook.com>
Date: Sun, 20 Dec 2020 14:55:44 +0100
Subject: [PATCH 13/14] Take out size=1 special case from MixtureSameFamily

---
 pymc3/distributions/mixture.py | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/pymc3/distributions/mixture.py b/pymc3/distributions/mixture.py
index dee5a21c66..51b35812a7 100644
--- a/pymc3/distributions/mixture.py
+++ b/pymc3/distributions/mixture.py
@@ -870,12 +870,6 @@ def random(self, point=None, size=None):
 
         # The `samples` array still has the `mixture_axis`, so we must remove it:
         output = samples[(..., 0) + (slice(None),) * len(event_shape)]
-
-        # Final oddity: if size == 1, pymc3 defaults to reducing the sample_shape dimension
-        # We do this to stay consistent with the rest of the package even though
-        # we shouldn't have to do it.
-        if size == 1:
-            output = output[0]
         return output
 
     def _distr_parameters_for_repr(self):

From 39ac694cc868c4be5ce5efece9ee1351ea9e57c2 Mon Sep 17 00:00:00 2001
From: Michael Osthege <m.osthege@fz-juelich.de>
Date: Sun, 20 Dec 2020 17:25:27 +0100
Subject: [PATCH 14/14] Use more professional terminology

Co-authored-by: Thomas Wiecki <thomas.wiecki@gmail.com>
---
 RELEASE-NOTES.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/RELEASE-NOTES.md b/RELEASE-NOTES.md
index ab3cb8621d..32ae60e90c 100644
--- a/RELEASE-NOTES.md
+++ b/RELEASE-NOTES.md
@@ -13,7 +13,7 @@ It also brings some dreadfully awaited fixes, so be sure to go through the chang
   - __`shape=(1,)` and `shape=1` now become vectors.__ Previously they were collapsed into scalars
   - 0-length dimensions are now ruled illegal for random variables and raise a `ValueError`.
 - In `sample_prior_predictive` the `vars` kwarg was removed in favor of `var_names` (see [#4327](https://github.com/pymc-devs/pymc3/pull/4327)).
-- Removed `theanof.set_theano_config` because it illegally touched Theano's privates (see [#4329](https://github.com/pymc-devs/pymc3/pull/4329)).
+- Removed `theanof.set_theano_config` because it illegally changed Theano's internal state (see [#4329](https://github.com/pymc-devs/pymc3/pull/4329)).
 
 ### New Features
 - `OrderedProbit` distribution added (see [#4232](https://github.com/pymc-devs/pymc3/pull/4232)).