Making things more functional in Python

Michael Hoffman at mh391.invalid
Fri Mar 4 17:49:37 CET 2005

gf gf wrote:
> Is there a better, more FP style, more Pythonic way to
> write this:
> def compute_vectors(samples, dset):
> 	vectors = {}
> 	for d in dset:
> 		vectors[d] = [sample.get_val(d) for sample in
> samples]
> 	return vectors
> Namely, I'd like to get rid of the initilization
> (vectors = {}) and also the loop

Generate the whole dictionary on the fly with a Python 2.4 generator

dict((d, [sample.get_val(d) for sample in samples]) for d in dset)

Whether this is "better" or not I think mainly hinges on which
one you ahve an easier time understanding later. Personally I would
prefer this version, but it's easy to get carried away trying to
functionalize things to the point that a procedural version is much
easier to understand.

> Yet, I'd hate to put an assignment into Python's FP list
 > comprehensions.

Indeed it's not possible to have an assignment in a list comprehension.
(Unless it's a side-effect due to a function called by the list

> Ideally, I'd like something like this:
> vectors.dict_add({d:result}) for [sample.get_val(d)
> for sample in samples for d in dset].  

You can't use the name "vectors" without first initializing it
Michael Hoffman

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