enabled type checking for io_pymc3 input

CHANGELOG.md

@@ -25,6 +25,7 @@
 * Fix `ess/rhat` plots in `plot_forest` ([1606](https://github.com/arviz-devs/arviz/pull/1606))
 * Fix `from_numpyro` crash when importing model with `thinning=x` for `x > 1` ([1619](https://github.com/arviz-devs/arviz/pull/1619))
 * Upload updated mypy.ini in ci if mypy copilot fails ([1624](https://github.com/arviz-devs/arviz/pull/1624))
+* Enforced using `pymc3.backends.base.MultiTrace` as the `trace` value for `io_pymc3`  ([1629](https://github.com/arviz-devs/arviz/pull/1629))
 
 ### Deprecation
 * Deprecated `index_origin` and `order` arguments in `az.summary` ([1201](https://github.com/arviz-devs/arviz/pull/1201))

arviz/data/io_pymc3.py

@@ -104,6 +104,12 @@ def __init__(
         # way to access the model from the trace.
         self.attrs = None
         if trace is not None:
+            if isinstance(self.trace, InferenceData):
+                raise ValueError(
+                    "Using the `InferenceData` as a `trace` argument won't work. "
+                    "Please use the `arviz.InferenceData.extend` method to extend the "
+                    "`InferenceData` with groups from another `InferenceData`."
+                )
             if self.model is None:
                 self.model = list(self.trace._straces.values())[  # pylint: disable=protected-access
                     0

arviz/tests/external_tests/test_data_pymc.py

@@ -139,6 +139,16 @@ def test_from_pymc_predictions(self, data, eight_schools_params):
             assert ivalues.shape[0] == 1  # one chain in predictions
             assert np.all(np.isclose(ivalues[0], values))
 
+    def test_from_pymc_trace_inference_data(self):
+        # check if error is raised successfully after passing InferenceData as trace
+        with pm.Model():
+            p = pm.Uniform("p", 0, 1)
+            pm.Binomial("w", p=p, n=2, observed=1)
+            trace = pm.sample(100, chains=2, return_inferencedata=True)
+            assert isinstance(trace, InferenceData)
+            with pytest.raises(ValueError):
+                from_pymc3(trace=trace)
+
     def test_from_pymc_predictions_new(self, data, eight_schools_params):
         # check creating new
         inference_data, posterior_predictive = self.make_predictions_inference_data(