Explanation of dictionary creation

Question

I am relatively new to Python, and I need some help to understand how the output is obtained for the following code:

keys = ['id', 'name', 'age']
values = [10, 'Ross', 19]
a_dict = {key:value for key in keys for value in values}
print(a_dict)

The output is:

{'id': 19, 'name': 19, 'age': 19}

I have tried nested loop too and I got the same output. I also tried interchanging key and value in the loop but there was no effect.

Can someone explain this please?

Edit:

I know how to get the output as

{'id': 10, 'name': 'Ross', 'age': 19}

I am only requesting an explanation for how the code I wrote works.. especially how the for loop works for the value part.

What do you expect as output? Dictionary keys can't be duplicated. — Austin, Jun 15 '19 at 05:26
Look at this: [Convert 2 lists into dictionary](https://stackoverflow.com/questions/209840/convert-two-lists-into-a-dictionary-in-python) — Shadowfax, Jun 15 '19 at 05:29
Possible duplicate of [Convert two lists into a dictionary in Python](https://stackoverflow.com/questions/209840/convert-two-lists-into-a-dictionary-in-python) — Anton vBR, Jun 15 '19 at 05:31
I have come from that page only.. I just tried this out and it didn't work, so I asked it here.. — a_faulty_star, Jun 15 '19 at 06:01

Reblochon Masque · Accepted Answer · 2019-06-15T06:10:00.990

4

You need to iterate simultaneously on both list, in order to pair the values with the keys:

keys = ['id', 'name', 'age']
values = [10, 'Ross', 19]
a_dict = {key:value for key, value in zip(keys, values)}
print(a_dict)

output:

{'id': 10, 'name': 'Ross', 'age': 19}

By comparison, the code you wrote a_dict = {key:value for key in keys for value in values} does:

iterates over the keys.
then, for each key, iterates over the values.
for each key, assigns each value in succession, each time overwriting the values already assigned, and terminating with the last value assigned to all the keys, that is:
'id': 10, 'name': 10, 'age': 10'
'id': 'Ross', 'name': 'Ross', 'age': 'Ross'
'id': 19, 'name': 19, 'age': 19'

edited Jun 15 '19 at 06:10

answered Jun 15 '19 at 05:28

Reblochon Masque

1

And in that case it would be as simple as: `dict(zip(keys, values))` – Anton vBR Jun 15 '19 at 05:29
1

Yes @AntonvBR, however, the most compact answer is not always conducive to explaining what is happening. – Reblochon Masque Jun 15 '19 at 05:31
I know the result of using zip, I just wanted to understand how the code I wrote works.. – a_faulty_star Jun 15 '19 at 06:03
1

You are welcome @a_faulty_star; I added a step by step explanation of what your code does. – Reblochon Masque Jun 15 '19 at 06:10

1 Answers1