Accessing list of Python objects by object attribute

Question

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.

Answer 1

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

Answer 2

You could create an instance.

apple = Fruit('apple', 'red')
print(apple.color) # I use Python 3.x

I'm not sure I'm following your question. But maybe this helps.

edit: in an attempt to gain my reputation back...

you could use instances WITHIN a dictionary. For example...

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

or if you prefer:

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

either way, you can call a fruit's color simply with

print(fruits['banana'].color)