Python Dictionary Comprehension Not Outputting As Expected

Question

I am playing around with dictionaries, and thought how would I create a dictionary using comprehensions. I thought

{k:v for k in [0,1,2] for v in [5,8,7]}

would print as

{0:5, 1:8, 2:7}

But instead it prints as

{0: 7, 1: 7, 2: 7}

Why is this happening and what modifications would I need to make to get the first output?

The keys 0, 1 & 2 will all be set to 5 on the first iteration, then 8 then 7 — , Oct 04 '21 at 18:44
@BrutusForcus Ah I see, so would it better to do this using the zip() function? — APM500, Oct 04 '21 at 18:45
you could try zipping like: {k: v for k, v in zip([0, 1, 2], [5, 8, 7])} — ihpar, Oct 04 '21 at 18:45

score 2 · Accepted Answer · answered Oct 04 '21 at 18:46

Your list comprehension is equivalent to nested loops:

result = {}
for v in [5, 8, 7]:
    for k in [0, 1, 2]:
        result[k] = v

So each iteration of the outer loop sets all the keys to that value, and at the end you have the last value in all of them.

Use zip() to iterate over two lists in parallel.

{k: v for k, v in zip([0, 1, 2], [5, 8, 7])}

You can also just use the dict() constructor:

dict(zip([0, 1, 2], [5, 8, 7]))

score 2 · Answer 2 · answered Oct 04 '21 at 18:46

Whenever you have trouble with a comprehension, unroll it into the equivalent loops. Which in this case goes:

mydict = {}
for v in [5,8,7]:
    for k in [0,1,2]:
        mydict[k] = v

Each successive assignment to mydict[k] overwrites the previous one.

2 Answers2