Dictionary (.values,.keys,.items) methods

Question

I'm getting a little ahead of myself in the course but I wondered if anyone could tell me why when using the methods above like so:

for key, value in my_dict.items():
    print('{}: {}'.format(key.title(), value))

We can gain access to the tuple in the view objects but when I try and do something like this:

dct = {
    'name': 'bob',
    'job': 'cook',
    'feet': 2,
    'loves': 'cake'
}

my_keys = []
my_keys = dct.keys()
my_keys[1]

I get the error "TypeError: 'dict_keys' object does not support indexing".

I can fix thix by converting it to an array like so:

...
my_keys.extend(dct.keys())

print(my_keys)
print(my_keys[1])

But where is the conversion in the for loop?

Answer 1 · 2016-03-26T16:37:45Z

March 26, 2016 4:37pm

Using the keys() method returns a special dict_keys object that cannot be indexed, but can be iterated over.

When iterated over, the keys are ordered according to their hashed values. So they won't be in a natural order, but will be deterministic meaning as long as the set of keys hasn't changed the order of iterated keys will be the same.

One way to get a list of keys is to use list() to iterate over the dict_key object

>>> dict1 = {'keys':0, 'in':1, 'a': 2, 'dict': 3, 'are': 4, 'hash': 5, 'ordered': 6}
>>> key_list = list(dict1.keys())
>>> key_list
['a', 'dict', 'ordered', 'in', 'keys', 'are', 'hash']
>>> key_list[0]
'a'

If the dict keys haven't changed, the list order will also be the same. This is true independent on the values stored in the dict. Changing values does not effect the hashed key order.

>>> dict1['a'] = "new value"
>>> list(dict1.keys())
['a', 'dict', 'ordered', 'in', 'keys', 'are', 'hash']

The for loop doesn't convert the keys to a list. Instead it iterates over the dict_keys object. Behind the scenes it does the following:

# staring with dict_keys object
>>> dict1_keys_obj = dict1.keys()

# the __iter__() method is called to return an iterable 
>>> dict1_keys_obj_iter = dict1_keys_obj.__iter__()

# each for-loop iteration, next() is used to get next item
>>> dict1_keys_obj_iter.__next__()
'a'
>>> dict1_keys_obj_iter.__next__()
'dict'
>>> dict1_keys_obj_iter.__next__()
'ordered'
>>> dict1_keys_obj_iter.__next__()
'in'

Answer 2 · 2016-03-26T15:01:18Z

March 26, 2016 3:01pm

Dictionaries have no specific order, so they cannot be indexed. But they can be iterated, which is what the loop is doing.

If indexing it did return a value, there's no way to know which item it would return. Similarly with the values if you were able to do this: dct.values()[2], but you couldn't predict which value you'd get. But you can do this: dct['job'], which in this case will always returrn 'cook'.

Welcome to the Treehouse Community

Looking to learn something new?

Nathan McGinley

Nathan McGinley

Dictionary (.values,.keys,.items) methods

2 Answers

Chris Freeman

Chris Freeman

Nathan McGinley

Nathan McGinley

Chris Freeman

Chris Freeman

Steven Parker

Steven Parker