# Shape issues when creating Dirichlet mixtures

**URL:** <https://discourse.pymc.io/t/shape-issues-when-creating-dirichlet-mixtures/7909>\
**Category:** Questions\
**Created:** [August 19, 2021, 3:18am UTC](https://discourse.pymc.io/t/shape-issues-when-creating-dirichlet-mixtures/7909 "2021-08-19T03:18:14Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![PedroSebe](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/pedrosebe/32/2843_2.png) [@PedroSebe](https://discourse.pymc.io/u/PedroSebe)\
**Post date:** [August 19, 2021, 3:18am UTC](https://discourse.pymc.io/t/shape-issues-when-creating-dirichlet-mixtures/7909/1 "2021-08-19T03:18:15Z")

</div>

I am having trouble writing models like this in PyMC3:

z\_i\sim\text{Categorical}(p)  
X\_i|z\_i\sim\text{Dirichlet}(\alpha\_{z\_i})

It’s a mixture model in which the components themselves are Dirichlet distributions. I studied this gaussian example from the documentation:

```auto
with pm.Model() as model:
    mu = pm.Normal('mu', mu=np.arange(5), sigma=1, shape=5) # Each component has an independent mean
    w = pm.Dirichlet('w', a=np.ones((3, 5))) # w is a stack of 3 independent 5 component weight arrays
    components = pm.Normal.dist(mu=mu, sigma=1, shape=(3, 5))
    # The mixture is an array of 3 elements.
    # Each can be thought of as an independent scalar mixture of 5 components
    like = pm.Mixture('like', w=w, comp_dists = components, shape=3)

```

I tried to replicate it as follows:

```auto
with pm.Model() as model:
  comp_dists = pm.Dirichlet.dist(np.ones((10,15)), shape=(10,15))
  weights = pm.Dirichlet("weights", np.ones((20, 10)), shape=(20,10))
  mix = pm.Mixture("likelihood", w=weights, comp_dists=comp_dists)

```

But it gives me ValueError: Input dimension mis-match. (input[0].shape[0] = 20, input[1].shape[0] = 10). I have tried exchanging the dimensions, using MixtureSameFamily instead of Mixture and using a list of dists instead os a multidimensional dist, but to no avail. Perhaps I am doing something silly with those shape arguments, but I’ve been stuck on this for a few days now.

Can you please provide some guidance? Thanks!