Python: Tuples/dictionaries as keys, select, sort

Question

Suppose I have quantities of fruits of different colors, e.g., 24 blue bananas, 12 green apples, 0 blue strawberries and so on. I'd like to organize them in a data structure in Python that allows for easy selection and sorting. My idea was to put them into a dictionary with tuples as keys, e.g.,

{     ('banana',    'blue' ): 24,     ('apple',     'green'): 12,     ('strawberry','blue' ): 0,     # ... } 

or even dictionaries, e.g.,

{     {'fruit': 'banana',    'color': 'blue' }: 24,     {'fruit': 'apple',     'color': 'green'}: 12,     {'fruit': 'strawberry','color': 'blue' }: 0,     # ... } 

I'd like to retrieve a list of all blue fruit, or bananas of all colors, for example, or to sort this dictionary by the name of the fruit. Are there ways to do this in a clean way?

It might well be that dictionaries with tuples as keys are not the proper way to handle this situation.

All suggestions welcome!

Answer 1

Personally, one of the things I love about python is the tuple-dict combination. What you have here is effectively a 2d array (where x = fruit name and y = color), and I am generally a supporter of the dict of tuples for implementing 2d arrays, at least when something like numpy or a database isn't more appropriate. So in short, I think you've got a good approach.

Note that you can't use dicts as keys in a dict without doing some extra work, so that's not a very good solution.

That said, you should also consider namedtuple(). That way you could do this:

>>> from collections import namedtuple >>> Fruit = namedtuple("Fruit", ["name", "color"]) >>> f = Fruit(name="banana", color="red") >>> print f Fruit(name='banana', color='red') >>> f.name 'banana' >>> f.color 'red' 

Now you can use your fruitcount dict:

>>> fruitcount = {Fruit("banana", "red"):5} >>> fruitcount[f] 5 

Other tricks:

>>> fruits = fruitcount.keys() >>> fruits.sort() >>> print fruits [Fruit(name='apple', color='green'),   Fruit(name='apple', color='red'),   Fruit(name='banana', color='blue'),   Fruit(name='strawberry', color='blue')] >>> fruits.sort(key=lambda x:x.color) >>> print fruits [Fruit(name='banana', color='blue'),   Fruit(name='strawberry', color='blue'),   Fruit(name='apple', color='green'),   Fruit(name='apple', color='red')] 

Echoing chmullig, to get a list of all colors of one fruit, you would have to filter the keys, i.e.

bananas = [fruit for fruit in fruits if fruit.name=='banana']