Applying a function recursively

hetchkay hetchkay at
Sat Sep 10 09:28:49 EDT 2011

> I suspect, if you can be explicit about the goal you're aiming for with
> this code, a better design can be found that doesn't require all those
> polymorphism-breaking type checks.
It is difficult to explain what I am trying to do, but let me try. I
am mapping data from one hierarchy into another. The mapping rules are
quite complex. I developed a system such that the rules could be
defined as "expressions" of the source hierarchy i.e a particular
entry in the target hierarchy could be represented as an expression of
entries in the source hierarchy. Suppose a point is stored in x, y
coordinates in the source hierarchy, and in polar coordinates in the
target hierarchy, I could write (forget for the moment what sourcePt
pointMapping = {
  sourcePt.key : dict(
     radius = Sqrt(sourcePt.value['x']**2 + sourcePt.value['y']**2),
     angle = Atan(sourcePt.value['y']/sourcePt.value['x']),
The above dictionary is delay-evaluated. sourcePt is an instance of a
class that facilitates the delayed evaluation. Sqrt, Atan etc. are
wrappers to the math functions to facilitate delayed evaluation. When
I encounter a point object, I could 'evaluate' the above mapping for
the point object to get the target dictonary.
The actual requirements are much more involved than this. The
motivation of the design was to enable application developers (who are
not experts in python) to be able to write the mappings. The mappings
also need to be readable.
You could consider this to be some sort of DSL. However, because of
the number of rules involved, I am trying to be as close to Python
expressions as possible. If the target setting is to be a tuple, for
example, I want to be able to write the tuple directly as "( expr1,
expr2 )", rather than, say, "Tuple(expr1, expr2)".
There is also a requirement to validate the mapping on load so that
run-time errors are minimized.
The system we are developing is quite reusable and we have been able
to use it for three different mappings so far. At this point, I am
trying to profile the code and noticed that a non-trivial amount of
time is being spent in the particular function I mentioned in this


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