CODEWITHSUNDEEP
python
Hiro Gotanda
Hiro Gotanda

Reputation: 129

Something I do not understand about dictionaries

When I was trying to understand Python dictionaries, I compared the output of two programs. I don't understand why the output is different.

Both programs start with 

data = {
        'key1': 'value1',
        'key2': 'value2',
        'key3': 'value3'
        }

First program:

for keys in data.items():
    print keys

Second program:

for keys, values in data.items():
    print keys, values

The outputs are 

('key3', 'value3')
('key2', 'value2')
('key1', 'value1')

and 

key3 value3
key2 value2
key1 value1

Why does the first output show parenthesised strings?

Why doesn't the second output show the commas?

Upvotes: 2

Views: 110

Answers (2)

Jon Clements
Jon Clements

Reputation: 142106

dict.items returns a sequence of 2-tuples, of (key, value).

What happens in the first example is that you're taking a single element from the that at at time, which in this case is the whole tuple (key, value). When you use for key, value in Python performs "unpacking" which means it assigns the first element of that tuple to key to key and the next element to value, so you can access them as separate variables.

When you print those, Python prints a tuple (your first example), as (1, 2), but when it's unpacked and you print two separate variables, it prints the number 1 followed by the number 2 with a space in between.

Upvotes: 7

zhangyangyu
zhangyangyu

Reputation: 8610

dict.items return a list of tuples of key, value pairs. If only one name in the for, it assigns the tuple to the name. With two names, it unpacks the key, value to the names separately. See:

>>> a, b = (1, 2)
>>> a
1
>>> b
2
>>> a = (1, 2)
>>> a
(1, 2)
>>>

Upvotes: 6

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