Merge pull request #2554 from quantopian/datashape-is-slow

PERF: Avoid slow exception repr in blaze.

zipline/pipeline/loaders/blaze/core.py

@@ -452,7 +452,13 @@ def _get_metadata(field, expr, metadata_expr, no_metadata_rule):
         return metadata_expr
 
     try:
-        return expr._child['_'.join(((expr._name or ''), field))]
+        # The error produced by expr[field_name] when field_name doesn't exist
+        # is very expensive. Avoid that cost by doing the check ourselves.
+        field_name = '_'.join(((expr._name or ''), field))
+        child = expr._child
+        if field_name not in child.fields:
+            raise AttributeError(field_name)
+        return child[field_name]
     except (ValueError, AttributeError):
         if no_metadata_rule == 'raise':
             raise ValueError(