# Custom multivariate density via DensityDist

I am struggling to define a custom distribution via DensityDist in PyMC v4. I want to implement a multivariate density, say f(x_0, x_1), but I’m not sure if I’m using the `shape` argument correctly.

Consider the following simplified example:

``````def f(x):
return x[0] + x[1]  # not an actual density

with pm.Model() as model:
custom_dist = pm.DensityDist("custom_dist", logp=f, shape=2)
``````

Will this code produce a 2-dimensional distribution `custom_dist` for x=[x_0,x_1]? Or will it somehow be interpreted as two independent scalar distributions? How can I check this to make sure?

This guide may help you: Distribution Dimensionality — PyMC dev documentation. You can use the same tools to reason about shapes of the density dist.

You might need to tell DensityDist that the support dimension is 1 (a vector) and, if you have any parameters, what the core dimensions of each are.

But in your very simple case is something not working?

Thanks for your reply @ricardoV94. I had indeed tried to specify `ndim_supp=1` following the dimensionality guide, but then I get an error I can’t seem to figure out. Specifically, an `IndexError: list index out of range` with the following error trace:

Error
``````---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Input In [170], in <cell line: 4>()
2     return x[0] + x[1]  # not an actual density
4 with pm.Model() as model:
----> 5     custom_dist = pm.DensityDist("custom_dist", logp=f, shape=2, ndim_supp=1)

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/pymc/distributions/distribution.py:809, in DensityDist.__new__(cls, name, logp, logcdf, random, moment, ndim_supp, ndims_params, dtype, *dist_params, **kwargs)
806     return moment(rv, size, *dist_params)
808 cls.rv_op = rv_op
--> 809 return super().__new__(cls, name, *dist_params, **kwargs)

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/pymc/distributions/distribution.py:263, in Distribution.__new__(cls, name, rng, dims, initval, observed, total_size, transform, *args, **kwargs)
259     raise TypeError(f"Name needs to be a string but got: {name}")
261 # Create the RV and process dims and observed to determine
262 # a shape by which the created RV may need to be resized.
--> 263 rv_out, dims, observed, resize_shape = _make_rv_and_resize_shape(
264     cls=cls, dims=dims, model=model, observed=observed, args=args, **kwargs
265 )
267 if resize_shape:
268     # A batch size was specified through `dims`, or implied by `observed`.
269     rv_out = change_rv_size(rv=rv_out, new_size=resize_shape, expand=True)

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/pymc/distributions/distribution.py:165, in _make_rv_and_resize_shape(cls, dims, model, observed, args, **kwargs)
162 """Creates the RV and processes dims or observed to determine a resize shape."""
163 # Create the RV without dims information, because that's not something tracked at the Aesara level.
164 # If necessary we'll later replicate to a different size implied by already known dims.
--> 165 rv_out = cls.dist(*args, **kwargs)
166 ndim_actual = rv_out.ndim
167 resize_shape = None

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/pymc/distributions/distribution.py:813, in DensityDist.dist(cls, *args, **kwargs)
811 @classmethod
812 def dist(cls, *args, **kwargs):
--> 813     output = super().dist(args, **kwargs)
814     if cls.rv_op.dtype == "floatX":
815         dtype = aesara.config.floatX

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/pymc/distributions/distribution.py:351, in Distribution.dist(cls, dist_params, shape, **kwargs)
346 create_size, ndim_expected, ndim_batch, ndim_supp = find_size(
347     shape=shape, size=size, ndim_supp=cls.rv_op.ndim_supp
348 )
349 # Create the RV with a `size` right away.
350 # This is not necessarily the final result.
--> 351 rv_out = cls.rv_op(*dist_params, size=create_size, **kwargs)
353 # Replicate dimensions may be prepended via a shape with Ellipsis as the last element:
354 if shape is not None and Ellipsis in shape:

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/aesara/tensor/random/op.py:279, in RandomVariable.__call__(self, size, name, rng, dtype, *args, **kwargs)
278 def __call__(self, *args, size=None, name=None, rng=None, dtype=None, **kwargs):
--> 279     res = super().__call__(rng, size, dtype, *args, **kwargs)
281     if name is not None:
282         res.name = name

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/aesara/graph/op.py:296, in Op.__call__(self, *inputs, **kwargs)
254 r"""Construct an `Apply` node using :meth:`Op.make_node` and return its outputs.
255
256 This method is just a wrapper around :meth:`Op.make_node`.
(...)
293
294 """
295 return_list = kwargs.pop("return_list", False)
--> 296 node = self.make_node(*inputs, **kwargs)
298 if config.compute_test_value != "off":
299     compute_test_value(node)

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/aesara/tensor/random/op.py:324, in RandomVariable.make_node(self, rng, size, dtype, *dist_params)
319 elif not isinstance(rng.type, RandomType):
320     raise TypeError(
321         "The type of rng should be an instance of either RandomGeneratorType or RandomStateType"
322     )
--> 324 shape = self._infer_shape(size, dist_params)
326 dtype = self.dtype or dtype

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/aesara/tensor/random/op.py:252, in RandomVariable._infer_shape(self, size, dist_params, param_shapes)
250     shape_supp = ()
251 else:
--> 252     shape_supp = self._supp_shape_from_params(
253         dist_params,
254         param_shapes=param_shapes,
255     )
257 shape = tuple(shape_ind) + tuple(shape_supp)
258 if not shape:

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/aesara/tensor/random/op.py:162, in RandomVariable._supp_shape_from_params(self, dist_params, **kwargs)
154 def _supp_shape_from_params(self, dist_params, **kwargs):
155     """Determine the support shape of a `RandomVariable`'s output given its parameters.
156
157     This does *not* consider the extra dimensions added by the `size` parameter
(...)
160     Defaults to `param_supp_shape_fn`.
161     """
--> 162     return default_supp_shape_from_params(self.ndim_supp, dist_params, **kwargs)

File ~/.micromamba/envs/bfr-py39/lib/python3.9/site-packages/aesara/tensor/random/op.py:73, in default_supp_shape_from_params(ndim_supp, dist_params, rep_param_idx, param_shapes)
71     raise ValueError("ndim_supp must be greater than 0")
72 if param_shapes is not None:
---> 73     ref_param = param_shapes[rep_param_idx]
74     return (ref_param[-ndim_supp],)
75 else:

IndexError: list index out of range
``````

That happens because by default Aesara tries to find the support shape (2,) by looking at the shape of the parameters. Your DensityDist has no parameters, so it fails to do so.

I actually don’t know if you can pass the `_supp_shape_from_params` function that would return this information to DensityDist to override the default. We could try to do this when shape is provided in PyMC as that is sufficient (Aesara uses size internally, not shape, so it never sees that (2,))

We should open am issue on PyMC GitHub for this.
Aesara error message could also be better.