Question

Short Version:

In Python is there a way to (cleanly/elegantly) say “Give me these 5 (or however many) properties of an object, and nothing else, as a dictionary”?

Longer Version:

Using the Javascript Underscore library, I can reduce an bunch of objects/dictionaries (in JS they’re the same thing) to a bunch of subsets of their properties like so:

var subsets = _(someObjects).map(function(someObject) {
    _(someObject).pick(['a', 'd']);
});

If I want to do the same thing with a Python object (not a dictionary) however it seems like the best I can do is use a list comprehension and manually set each property:

subsets = [{"a": x.a, "d": x.d} for x in someObjects]

That doesn’t look so bad when there’s only two properties, and they’re both one letter, but it gets uglier fast if I start having more/longer properties (plus I feel wrong whenever I write a multi-line list comprehension). I could turn the whole thing in to a function that uses a for loop, but before I do that, is there any cool built-in Python utility thing that I can use to do this as cleanly (or even more cleanly) than the JS version?

Answer 1

This can be done simply by combining a list comprehension with a dictionary comprehension.

subsets = [{attr: getattr(x, attr) for attr in ["a", "d"]} 
           for x in someObjects]

Naturally, you could distill out that comprehension if you wanted to:

def pick(*attrs):
    return {attr: getattr(x, attr) for attr in attrs} 

subsets = [pick("a", "d") for x in someObjects]