Hi @davekch,
Welcome to the ROOT forum!
This is because column is not in scope by the time the expression is evaluated.
In principle, only global variables may be used in Python (in C++ you should be able to provide a lambda which captures some or all the variables in the scope where it is defined).
Also, see this related forum topic: RDataFrame: upload external variables to the Define.
FYI, @eguiraud.
Cheers,
J.