consolidated some batch caching code

pliers/transformers/base.py

@@ -236,18 +236,13 @@ def _iterate(self, stims, validation='strict', *args, **kwargs):
         results = []
         for batch in progress_bar_wrapper(batches):
             use_cache = config.get_option('cache_transformers')
-            target_inds = []
+            target_inds = {}
             non_cached = []
-            transformed_keys = set()
             for stim in batch:
                 key = hash((hash(self), hash(stim)))
-                if use_cache and (key in _cache or key in transformed_keys):
-                    target_inds.append(-1)  # signals to query cache
-                else:
-                    target_inds.append(len(non_cached))
+                if not (use_cache and (key in _cache or key in target_inds)):
+                    target_inds[key] = len(non_cached)
                     non_cached.append(stim)
-                    # Can't use _cache in case _transform fails
-                    transformed_keys.add(key)
 
             if len(non_cached) > 0:
                 batch_results = self._transform(non_cached, *args, **kwargs)
@@ -256,17 +251,17 @@ def _iterate(self, stims, validation='strict', *args, **kwargs):
 
             for i, stim in enumerate(batch):
                 key = hash((hash(self), hash(stim)))
-                if target_inds[i] == -1:
-                    results.append(_cache[key])
-                else:
-                    result = batch_results[target_inds[i]]
+                if key in target_inds:
+                    result = batch_results[target_inds[key]]
                     result = _log_transformation(stim, result, self)
                     self._propagate_context(stim, result)
                     if use_cache:
                         if isgenerator(result):
                             result = list(result)
                         _cache[key] = result
                     results.append(result)
+                else:
+                    results.append(_cache[key])
         return results
 
     def _transform(self, stim, *args, **kwargs):