# A problem interpreting an error in PyMC

**URL:** <https://discourse.pymc.io/t/a-problem-interpreting-an-error-in-pymc/14550>\
**Category:** General\
**Created:** [June 5, 2024, 12:53pm UTC](https://discourse.pymc.io/t/a-problem-interpreting-an-error-in-pymc/14550 "2024-06-05T12:53:45Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![Andre](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/andre/32/6205_2.png) [@Andre](https://discourse.pymc.io/u/Andre)\
**Post date:** [June 5, 2024, 12:53pm UTC](https://discourse.pymc.io/t/a-problem-interpreting-an-error-in-pymc/14550/1 "2024-06-05T12:53:45Z")

</div>

Good morning,

It’s been a long time since I last used PyMC and I realize that I have mostly forgotten how programs should be properly written!

To address a `Gamma-Poisson` problem that interests me, I was inspired by the question asked and addressed here: [HERE](https://discourse.pymc.io/t/pb-with-poisson-gamma/7548)

So, I adjusted the instructions given by **jlindbloom** a bit, based on what I remember, and figured I could write the program below:

```auto
import pymc as pm
import arviz as az

with pm.Model() as model:
    gamma = pm.Gamma('gamma', alpha=1, beta=1) # Prior
    p_obs = pm.Poisson('p_obs', mu=gamma, observed=[1]) # Likelihood

with model:
    idata = pm.sample(tune=1000,draws=5000,target_accept=0.9)
    idata_prior = pm.sample_prior_predictive(var_names=["gamma"],
                                          return_inferencedata=True)
    idata_posterior = pm.sample_posterior_predictive(idata, var_names=["p_obs"],
                                                     return_inferencedata=True,
                                                     extend_inferencedata=True,
                                                     predictions=False)

idata.extend(idata_prior)
idata.extend(idata_posterior)
    
az.plot_ppc(idata_prior, var_names=["p_obs"])      
az.plot_ppc(idata_posterior, var_names=["gamma"])

```

Unfortunately, what I feared happened, in the form of the error:

```auto
TypeError Traceback (most recent call last)
Cell In[22], line 20
     16 idata.extend(idata_prior)
     17 idata.extend(idata_posterior)
---> 20 az.plot_ppc(idata_prior, var_names=["p_obs"])   
     22 az.plot_ppc(idata_posterior, var_names=["gamma"])
...
TypeError: `data` argument must have the group "posterior_predictive" for ppcplot.

```

and, obviously, I completely forgot what the meaning of: `must have the group "posterior_predictive" for ppcplot` is and how to fix that…

I come to you for some advice and ask you to bear with me on this not-so-smart question; I’ll try to find some notes I must have taken on the subject.

---

<div class="post-metadata">

**Author:** ![ricardoV94](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/ricardov94/32/5775_2.png) [@ricardoV94](https://discourse.pymc.io/u/ricardoV94)\
**Post date:** [June 5, 2024, 1:17pm UTC](https://discourse.pymc.io/t/a-problem-interpreting-an-error-in-pymc/14550/2 "2024-06-05T13:17:15Z")

</div>

I think you can’t pass the prior dataset to az.plot\_ppc, since it is supposed to work with posterior predictive datasets, and not priors

---

<div class="post-metadata">

**Author:** ![ricardoV94](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/ricardov94/32/5775_2.png) [@ricardoV94](https://discourse.pymc.io/u/ricardoV94)\
**Post date:** [June 5, 2024, 1:19pm UTC](https://discourse.pymc.io/t/a-problem-interpreting-an-error-in-pymc/14550/3 "2024-06-05T13:19:57Z")

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Nevermind, arviz docs says it works with prior\_predictive as well. The error message does not reflect this. Try to call `pm.sample_prior_predictive` without setting `var_names` (or include `p_obs` as well) so that it also samples the observed variable. `plot_ppc` probably needs that one to not be empty

---

<div class="post-metadata">

**Author:** ![Andre](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/andre/32/6205_2.png) [@Andre](https://discourse.pymc.io/u/Andre)\
**Post date:** [June 5, 2024, 3:35pm UTC](https://discourse.pymc.io/t/a-problem-interpreting-an-error-in-pymc/14550/4 "2024-06-05T15:35:07Z")

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Thanks `ricardoV94`, but something still remains unclear ; please look:

` arviz.plot_ppc` contains (among other arguments):`data`, `var_names` and `group`.

- `data` is an object containing the observed and posterior/prior predictive data  
I understand the sentence as “and posterior predictive OR prior predictive data”
- `var_names` is optional (if `None` all variable are plotted)
- `group` may be “prior” or “posterior”, and there is no mention of group “posterior\_predictive”…

Running my program **without** the 2 lines calling to `az.plot_ppc` leads to:

**idata:**  
• posterior  
• posterior\_predictive  
• sample\_stats  
• prior  
• observed\_data  
**idata\_prior:**  
• prior  
• observed\_data  
**idata\_posterior:**  
• posterior  
• posterior\_predictive  
• sample\_stats  
• prior  
• observed\_data

Now, running the 2 lines:

```auto
az.plot_ppc(idata_prior, var_names=None)      
az.plot_ppc(idata_posterior, var_names=None)

```

still leads to the same error, (apparently linked to the line: `az.plot_ppc(idata_prior, var_names=None)`):

```auto
TypeError Traceback (most recent call last)
---> 20 az.plot_ppc(idata_prior, var_names=None)
TypeError: `data` argument must have the group "posterior_predictive" for ppcplot

```

But I don’t understand what this “posterior\_predictive” group has to do with the ` idata_prior` results. So, finally, what is clearly inaccurate / wrong in this program I made, what are the things I obviously forgot to include, and how could I correct it ?

Many thanks for any indication.

---

<div class="post-metadata">

**Author:** ![ricardoV94](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/ricardov94/32/5775_2.png) [@ricardoV94](https://discourse.pymc.io/u/ricardoV94)\
**Post date:** [June 5, 2024, 5:50pm UTC](https://discourse.pymc.io/t/a-problem-interpreting-an-error-in-pymc/14550/5 "2024-06-05T17:50:32Z")

</div>

I meant `var_names` in the call to `pm.sample_prior_predictive`. Note that there is no `prior_predictive` group in your `idata_prior` (only a `prior` group), because you didn’t sample any observed variables in `pm.sample_prior_predictive`.

---

<div class="post-metadata">

**Author:** ![Andre](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/andre/32/6205_2.png) [@Andre](https://discourse.pymc.io/u/Andre)\
**Post date:** [June 5, 2024, 7:26pm UTC](https://discourse.pymc.io/t/a-problem-interpreting-an-error-in-pymc/14550/6 "2024-06-05T19:26:06Z")

</div>

Hi `ricardoV94`, I’m sorry; I feel completely obtuse and can’t understand what’s going on.

I know I’m exaggerating by asking this, but could you do me a favor?

I tried each of these three lines:

```auto
    idata_prior = pm.sample_prior_predictive(var_names=["gamma", "p_obs"],
                                             return_inferencedata=True) 
    idata_prior = pm.sample_prior_predictive(var_names=None,
                                             return_inferencedata=True) 
    idata_prior = pm.sample_prior_predictive(return_inferencedata=True)

```

Each of them led to the same error (as already mentioned). Now, I am really stuck with the kind of instruction I should use…

Could you elaborate a bit more and give me more hints on what the right solution in order to get rid of this error?

I really don’t know how to move forward in this matter.

Thanks in advance