Generators and propagation of exceptions

r nbs.public at gmail.com
Fri Apr 8 17:55:08 CEST 2011


I had a problem for which I've already found a "satisfactory"
work-around, but I'd like to ask you if there is a better/nicer
looking solution. Perhaps I'm missing something obvious.

The code looks like this:

stream-of-tokens = token-generator(stream-of-characters)
stream-of-parsed-expressions = parser-generator(stream-of-tokens)
stream-of-results = evaluator-generator(stream-of-parsed-expressions)

each of the above functions consumes and implements a generator:

def evaluator-generator(stream-of-tokens):
  for token in stream-of-tokens:
    try:
       yield token.evaluate()           # evaluate() returns a Result
    except Exception as exception:
       yield ErrorResult(exception) # ErrorResult is a subclass of Result

The problem is that, when I use the above mechanism, the errors
propagate to the output embedded in the data streams. This means, I
have to make them look like real data (in the example above I'm
wrapping the exception with an ErrorExpression object) and raise them
and intercept them again at each level until they finally trickle down
to the output. It feels a bit like writing in C (checking error codes
and propagating them to the caller).

OTOH, if I don't intercept the exception inside the loop, it will
break the for loop and close the generator. So the error no longer
affects a single token/expression but it kills the whole session. I
guess that's because the direction flow of control is sort of
orthogonal to the direction of flow of data.

Any idea for a working and elegant solution?

Thanks,

-r



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