Question

What is the easiest/most elegant way to do the following in python:

def piecewiseProperty(aList):
    result = []
    valueTrue = 50
    valueFalse = 10

    for x in aList:
        if hasProperty(x):
            result.append(valueTrue)
        else
            result.append(valueFalse)

    return result

where hasProperty is some function with boolean return value.

One shorter (but opaque, and possibly less efficient) R-like way to do it would be this

trueIndexSet = set([ ind for ind,x in enumerate(aList) if hasProperty(x) ])
falseIndexSet = set(range(0:len(aList)).difference(trueIndexSet)
vals = sorted( [ (ind,10) for ind in falseIndexSet ] + [ (ind,50) for ind in trueIndexSet ] )
[ x for ind,x in vals]

Another much tidier approach would use dictionary lookup:

[ {True:50, False:10}[hasProperty(x)] for x in aList ]

Is there some clever and readable one-liner or built in function for doing this? It would basically be an if…else list comprehension.

Application of this question:
Just in case it’s of interest, I am using this to assign sizes to nodes in a network so that they are drawn differently. I want to draw nodes named with prefix “small_” size 10 and draw the other nodes size 50. NetworkX and pygraphviz can alter the sizes of the nodes by accepting a list of sizes, one for each node.

Answer 1

Use a conditional expression (pep-308):

[50 if hasProperty(x) else 10 for x in alist]