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Warning: Modifying a built-in function is not advised As @m_goldberg already stated, Lookup has Attributes HoldAllComplete, so a workaround will be to remove this Attribute: Edit: As per m_goldberg's recommendation attr = Attributes[Lookup]; Attributes[Lookup] = {}; Now t1 = TempHead[a -> 1, b -> 2, c -> 3]; t2 = TempHead[c -> 3, d -> 4, ...


Lookup has the attribute HoldAllComplete, which means the kernel evaluator will not see its arguments and, therefore, will not look at its up-values.


[Added: Virtually the same issue came up on StackOverflow a few years ago: Why do I have to evaluate this twice?] I would not consider it a bug if the manipulation of System` variables do no go the way you want. It turns out that the definition of TensorRank is not loaded until it is first evaluated. That initialization process resets the attributes of ...


As @Mr.Wizard asked, "...it's time for an answer, don't you think?". I forgot about this question, but after year it occurred to me to formulate an answer following @LeonidShifin suggestions: First get some test data: testData = FinancialData["GE", "Jan. 1, 2000"][[1 ;; 10]]; dataFile = "/Users/jagra/Data.test.csv"; Export[dataFile, testData, "CSV"]; ...

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