Don't require posterior in loo if not used

arviz/stats/stats.py

@@ -484,14 +484,12 @@ def loo(data, pointwise=False, reff=None, scale="deviance"):
 
     """
     inference_data = convert_to_inference_data(data)
-    for group in ("posterior", "sample_stats"):
-        if not hasattr(inference_data, group):
-            raise TypeError(
-                "Must be able to extract a {group} group from data!".format(group=group)
-            )
+    if not hasattr(inference_data, "sample_stats"):
+        raise TypeError(
+            "Must be able to extract a sample_stats group from data!"
+        )
     if "log_likelihood" not in inference_data.sample_stats:
         raise TypeError("Data must include log_likelihood in sample_stats")
-    posterior = inference_data.posterior
     log_likelihood = inference_data.sample_stats.log_likelihood
     log_likelihood = log_likelihood.stack(sample=("chain", "draw"))
     shape = log_likelihood.shape
@@ -508,6 +506,11 @@ def loo(data, pointwise=False, reff=None, scale="deviance"):
         raise TypeError('Valid scale values are "deviance", "log", "negative_log"')
 
     if reff is None:
+        if not hasattr(inference_data, "posterior"):
+            raise TypeError(
+                "Must be able to extract a posterior group from data!"
+            )
+        posterior = inference_data.posterior
         n_chains = len(posterior.chain)
         if n_chains == 1:
             reff = 1.0