Fixed Dirichlet.random returning output of wrong shapes

RELEASE-NOTES.md

@@ -35,6 +35,7 @@ It also brings some dreadfully awaited fixes, so be sure to go through the chang
 - Update the `logcdf` method of several continuous distributions to return -inf for invalid parameters and values, and raise an informative error when multiple values cannot be evaluated in a single call. (see [4393](https://github.com/pymc-devs/pymc3/pull/4393))
 - Improve numerical stability in `logp` and `logcdf` methods of `ExGaussian` (see [#4407](https://github.com/pymc-devs/pymc3/pull/4407))
 - Issue UserWarning when doing prior or posterior predictive sampling with models containing Potential factors (see [#4419](https://github.com/pymc-devs/pymc3/pull/4419))
+- Dirichlet distribution's `random` method is now optimized and gives outputs in correct shape (see [#4416](https://github.com/pymc-devs/pymc3/pull/4407))
 
 ## PyMC3 3.10.0 (7 December 2020)
 

pymc3/distributions/multivariate.py

@@ -471,33 +471,11 @@ def __init__(self, a, transform=transforms.stick_breaking, *args, **kwargs):
 
         super().__init__(transform=transform, *args, **kwargs)
 
-        self.size_prefix = tuple(self.shape[:-1])
         self.a = a = tt.as_tensor_variable(a)
         self.mean = a / tt.sum(a)
 
         self.mode = tt.switch(tt.all(a > 1), (a - 1) / tt.sum(a - 1), np.nan)
 
-    def _random(self, a, size=None):
-        gen = stats.dirichlet.rvs
-        shape = tuple(np.atleast_1d(self.shape))
-        if size[-len(shape) :] == shape:
-            real_size = size[: -len(shape)]
-        else:
-            real_size = size
-        if self.size_prefix:
-            if real_size and real_size[0] == 1:
-                real_size = real_size[1:] + self.size_prefix
-            else:
-                real_size = real_size + self.size_prefix
-
-        if a.ndim == 1:
-            samples = gen(alpha=a, size=real_size)
-        else:
-            unrolled = a.reshape((np.prod(a.shape[:-1]), a.shape[-1]))
-            samples = np.array([gen(alpha=aa, size=1) for aa in unrolled])
-            samples = samples.reshape(a.shape)
-        return samples
-
     def random(self, point=None, size=None):
         """
         Draw random values from Dirichlet distribution.
@@ -516,7 +494,10 @@ def random(self, point=None, size=None):
         array
         """
         a = draw_values([self.a], point=point, size=size)[0]
-        samples = generate_samples(self._random, a=a, dist_shape=self.shape, size=size)
+        output_shape = to_tuple(size) + to_tuple(self.shape)
+        a = broadcast_dist_samples_to(to_shape=output_shape, samples=[a], size=size)[0]
+        samples = stats.gamma.rvs(a=a, size=output_shape)
+        samples = samples / samples.sum(-1, keepdims=True)
         return samples
 
     def logp(self, value):

pymc3/tests/test_distributions_random.py

@@ -542,6 +542,24 @@ def test_probability_vector_shape(self):
         assert pm.Categorical.dist(p=p).random(size=4).shape == (4, 3, 7)
 
 
+class TestDirichlet(SeededTest):
+    @pytest.mark.parametrize(
+        "shape, size",
+        [
+            ((2), (1)),
+            ((2), (2)),
+            ((2, 2), (2, 100)),
+            ((3, 4), (3, 4)),
+            ((3, 4), (3, 4, 100)),
+            ((3, 4), (100)),
+            ((3, 4), (1)),
+        ],
+    )
+    def test_dirichlet_random_shape(self, shape, size):
+        out_shape = to_tuple(size) + to_tuple(shape)
+        assert pm.Dirichlet.dist(a=np.ones(shape)).random(size=size).shape == out_shape
+
+
 class TestScalarParameterSamples(SeededTest):
     def test_bounded(self):
         # A bit crude...