Pass conditional expression as function argument

Question

I'm writing a function for region growing on an irregular grid: Start in a seed bin, find the neighboring bins, and include them in the set of returned bin indices if a certain requirement is fulfilled. I would like for the requirement to be arbitrary, e.g., '> 2', '>= 2', '== 2', etc.. So the syntax would be:

myregion = get_region(bin_coordinates, bin_values, seed_bin, '> 2')

and the region of neighboring bins, whose values are greater than 2 will be returned.

Essentially I'm looking for the same functionality as in the pandas library, where the following is possible when querying a HDF store:

store.select('dfq',where="A>0 or C>0")

I could of course write some sort of parser with if/else statements to translate '> 2' into code, but I was wondering if there is a more elegant way?

pass in a lambda (an anonymous function), e.g. lambda x: x>2. Declare getregion as e.g. getregion(coordinates,values,testfun), and call it by e.g. getregion(thesecoords,thesevalues,lambda x: x>2), then in getregion use the test by e.g. if testfun(value). Choose a better name than testfun, though, one that describes what a result of True means. — DisappointedByUnaccountableMod
– DisappointedByUnaccountableMod, Commented Dec 8, 2015 at 9:28

DisappointedByUnaccountableMod · Accepted Answer · 2015-12-08 09:33:18Z

1

Use a lambda (an anonymous function), e.g. lambda x: x>2. Declare getregion as e.g. getregion(coordinates,values,testfun), and call it by e.g. getregion(thesecoords,thesevalues,lambda x: x>2), then in getregion use the test by e.g. if testfun(value). Choose a better name than testfun, though, one that describes what a result of True means.

Another approach would be to define some 'standard' evaluation functions and pass their name to getregion, e.g.

def gt2(x):
    return (x>2)

declare getregion as for lambda example:

def getregion( coordinates, values, testfun ):
    ...
    if testfun(x):
        ...

and call getregion like this:

getregion(thesecoordinates, thesevalues, gt2 )

HTH Barny

answered Dec 8, 2015 at 9:33

DisappointedByUnaccountableMod

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2 Comments

sulkeh Over a year ago

Thanks, that's a good solution. Still wonder wonder how the pandas functionality is implemented though...

DisappointedByUnaccountableMod Over a year ago

They have implemented a conditional expression evaluator - in the pandas source search for 'where=' in pytables.py. I guess the detailed implementation is in there somewhere...

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