How to unpack nested tuples in python?

Question

Let's say I have users (John, Anna) that can order fruits (oranges, apples) in a quantity of their choice.

Here is what I have done in python :

orders = (('John', (('apples', 3), ('oranges', 1))),
          ('Anna', (('oranges', 6))))

I'd like to iterate over the orders :

for user, order in orders:
    for fruit, quantity in order:
        print "%s wants %d %s" % (user, quantity, fruit)

This is the output i'm expecting :

John wants 3 apples
John wants 1 oranges
Anna wants 6 oranges

I get a ValueError: too many values to unpack. What am I doing wrong? Should I use something else than tuples? Is it my iteration that is bad?

Answer 1

It would have been useful to show the exact traceback.

The error is simply that Anna's order isn't actually a nested tuple: it's a simple tuple. You are missing a comma at the end:

orders = (('John', (('apples', 3), ('oranges', 1))),
          ('Anna', (('oranges', 6),)))
#                                 ^ here

Remember it's the comma that makes the tuple, not the parentheses.

As others have stated though, nested dicts would be more appropriate here.

Answer 2

Daniel has given an excellent answer to:

What am I doing wrong?

But to address your second question:

Should I use something else than tuples?

I would say that this is a much better fit for a dictionary:

>>> orders = {'John': {'apples': 3, 'oranges': 1}, 'Anna': {'oranges': 6}}
>>> for user, order in orders.items():
        for fruit, quantity in order.items():
            print "{0} wants {1} {2}".format(user, quantity, fruit)


Anna wants 6 oranges
John wants 1 oranges
John wants 3 apples

(Note that dictionaries are not guaranteed to retain the order of their contents - if the order is crucial, there is an alternative collections.OrderedDict that guarantees retention of insertion order.)

This is particularly useful to answer questions like "how many apples did John want?"

>>> orders['John']['apples']
3

which can be altered to guard against someone not having ordered that fruit, for example if you want to find out "how many apples were ordered in total?"

>>> sum(order.get('apples', 0) for order in orders.values())
3