Only calculate bindings (ctxt_obj and ctxt_dict) once for expr slots #29
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This PR deals with issue #27, and only addresses point 1 in that issue. It addresses a performance issue with expr slots in a slot_derivation. Point 1 was that the bindings for the expr are recalcualted any time an expr slot is found, and the recalculation occurs even if the bindings are the same. This is found in ObjectTransformer.map_object() where expr slots are handled. The bindings should only be calculated once. To deal with this I added the variables ctxt_obj and ctxt_dict (initialized to None) outside of the slot_derivations loop in ObjectTransformer.map_object. When an expr slot is encountered, the bindings (ctxt_obj and ctxt_dict) are only calculated if ctxt_obj is None, and the results assigned to ctxt_obj and ctxt_dict to be reused in a future iteration of the loop.
A more complex solution that deals with both points 1 and 2 in the issue is found in expr-opt-a (the second point being that only bindings that are required should be calculated, instead of all bindings, which is helpful performance-wise for very wide datasets).
Fixes #27