Retrieving single dictionary element in Python [duplicate]

Question

I want to retrieve only the fourth item in the dictionary "e" (below).

I tried using the OrderedDict() method, but it didn't work. Here are my results:

from collections import OrderedDict

e = OrderedDict()
e = {'a': 'A',
     'b': 'B',
     'c': 'C',
     'd': 'D',
     'e': 'E'
    }

for k, v in e.items():
    print k, v

print e.items()[3]

The last line returns: ('e', 'E')

So I turned the keys and values into lists, but here's how the lists appeared when I printed them:

['a', 'c', 'b', 'e', 'd']


['A', 'C', 'B', 'E', 'D']

For me, this explained why it happened, but not how it happened.

So, next I sorted them. That gave me the results I was looking for -- but it seemed unnecessarily complicated:

e = {'a': 'A',
     'b': 'B',
     'c': 'C',
     'd': 'D',
     'e': 'E'
}
k, v = sorted(e.keys()), sorted(e.values())
print "{}: {}".format(k[3], v[3])

Result: d: D

OrderedDict() wasn't necessary.

Is there an easier way to do this? And can someone explain why the elements in the dictionary are ordered like this:

keys: 'a', 'c', 'b', 'e', 'd'


values: 'A', 'C', 'B', 'E', 'D'

... which defies the structure of my original dictionary?

Answer 1

You're not using an ordered dict.

e = OrderedDict()
e = {'a': 'A',
     'b': 'B',
     'c': 'C',
     'd': 'D',
     'e': 'E'
    }

The first line creates an OrderedDict. The second line throws it away and replaces it with a regular dict, which is unordered. (Python variables don't have types.)

But you can't just do this:

e = OrderedDict({'a': 'A', ...})

...because that's still a regular dict, which is still unordered, and OrderedDict can't magically recreate your original source ordering.

Try this:

e = OrderedDict([('a', 'A'), ('b', 'B'), ...])

Now you should have a dict-like object with the ordering you want.

And can someone explain why the elements in the dictionary are ordered like this ... which defies the structure of my original dictionary?

Because dictionaries are unordered. They're just hash maps, and hash maps have no inherent ordering.

---

Note that you could also do this, which will preserve the pairing of keys and values (whereas your separate sortings will not):

print sorted(e.items())[3]