Accessing list of Python objects by object attribute

python

Solution

class Fruit:
    def __init__(self, name, color):
        self.name = name
        self.color = color

fruits = dict(zip(['apple', 'banana'], [Fruit('apple', 'red'), Fruit('banana', 'yellow')]))

print("An apple is %s" % fruits['apple'].color)

OR:

fruits = {fruit.name : fruit for fruit in [Fruit('apple', 'red'), Fruit('banana', 'yellow')]}

print("An apple is %s" % fruits['apple'].color)

The following does infact produce a set:

fruits = {Fruit('apple', 'red'), Fruit('banana', 'yellow')}

Note the difference from the way I created the dict

fruits = [Fruit('apple', 'red'), Fruit('banana', 'yellow')]

print("An apple is %s" % fruits[fruits.index('apple')].color)

Doesn't work because your list contains Objects of type fruit not strings, and that is the same story here:

fruits = FruitList([Fruit('apple', 'red'), Fruit('banana', 'yellow')])

print("An apple is %s" % fruits['apple'].color)

To make the above method work, do the following:

class Fruit:
    def __init__(self, name, color):
        self.name = name
        self.color = color

    def __eq__(self, other):
        if isinstance(other, Fruit):
            return (self.name, self.color) == (other.name, other.color)
        return self.name == other

fruits = [Fruit('apple', 'red'), Fruit('banana', 'yellow')]
print("An apple is %s" % fruits[fruits.index('apple')].color)

I don't recommend this because in the case that your list happens to contain the string `apple`, then attribute call to `color` becomes invalid because strings do not have an attribute called `color`

Problem

I don't know if what I am trying to do is so un-Pythonic that I'm simply trying to do it wrong, or if I don't know how to ask the question properly. It makes sense to me to be able to do this, but I have searched 15 different ways and can't find the answer. What I want to do seems so simple: I have a list of objects. I want to access that list by a property of the objects. This code works: ``` class Fruit: def __init__(self, name, color): self.name = name self.color = color def __str__(self): return self.name class BaseballPlayer: def __init__(self, name, number, position): self.name = name self.number = number self.position = position def __str__(self): return self.name class ListByProperty(list): def __init__(self, property, list): super(ListByProperty, self).__init__(list) self.property = property def __getitem__(self, key): return [item for item in self if getattr(item, self.property) == key][0] fruits = ListByProperty('name', [Fruit('apple', 'red'), Fruit('banana', 'yellow')]) baseballTeam = ListByProperty('position', [BaseballPlayer('Greg Maddux', 31, 'P'), BaseballPlayer('Javy Lopez', 8, 'C')]) teamByNumber = ListByProperty('number', baseballTeam) print 'Apples are', fruits['apple'].color pitcher = baseballTeam['P'] print 'The pitcher is #%s, %s' % (pitcher.number, pitcher) print '#8 is', teamByNumber[8] >>> Apples are red The pitcher is #31, Greg Maddux #8 is Javy Lopez ``` But do I really have to make my own list class to do something this simple? Is there no generic way other than looping or a listcomp? This seems like it should be a very common thing to do, to have a list of objects and access items in the list by a property of the objects. It seems like it should be commonly supported in a way similar to `sorted(key=...)`. Note that this is not the same case as needing a dict. In fact, the whole point of using a list of objects instead of a dict is to avoid having to do something like: ``` fruits = {'apple': Fruit('apple', 'red')} ``` ...which requires you to type `apple` twice. It seems like there should be a generic way to do something like this: ``` print 'Apples are', fruits['apple'].color ``` ...without having to subclass `list`. And okay, you can build a dict like this: ``` fruits = [Fruit('apple', 'red'), Fruit('banana', 'yellow')] fruits = {f.name: f for f in fruits} ``` Or you can one-line it, but that still seems...uh...syntactically sour? :) The best way I've figured out so far is: ``` class DictByProperty(dict): def __init__(self, property, list): super(DictByProperty, self).__init__({getattr(i, property): i for i in list}) self.property = property fruits = DictByProperty('name', [Fruit('apple', 'red')]) ``` Oh well, thanks, I've learned a lot already from this question.

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