added some comments to batch caching

pliers/transformers/base.py

@@ -240,17 +240,21 @@ def _iterate(self, stims, validation='strict', *args, **kwargs):
             non_cached = []
             for stim in batch:
                 key = hash((hash(self), hash(stim)))
+                # If using the cache, only transform stims that aren't in the
+                # cache and haven't already appeared in the batch
                 if not (use_cache and (key in _cache or key in target_inds)):
                     target_inds[key] = len(non_cached)
                     non_cached.append(stim)
 
+            # _transform will likely fail if given an empty list
             if len(non_cached) > 0:
                 batch_results = self._transform(non_cached, *args, **kwargs)
             else:
                 batch_results = []
 
             for i, stim in enumerate(batch):
                 key = hash((hash(self), hash(stim)))
+                # Use the target index to get the result from batch_results
                 if key in target_inds:
                     result = batch_results[target_inds[key]]
                     result = _log_transformation(stim, result, self)
@@ -260,6 +264,7 @@ def _iterate(self, stims, validation='strict', *args, **kwargs):
                             result = list(result)
                         _cache[key] = result
                     results.append(result)
+                # Otherwise, the result should be in the cache
                 else:
                     results.append(_cache[key])
         return results