List comprehension from dict of lists

Question

I have a dict-

a = {'b': [1,2,3], 'c':[4,5,6]}

I want to use list comprehension only to achieve this output-

[['c', 4], ['c', 5], ['c', 6], ['b', 1], ['b', 2], ['b', 3]]

A simple for loop gets it done with -

x = []
for k, v in a.iteritems():
    for i in v:
        x.append([k, i])

Tried to convert it to list comprehension, I did this-

[[k,i] for i in v for k, v in a.items()]

But weirdly for me, I got an output

[['c', 1], ['b', 1], ['c', 2], ['b', 2], ['c', 3], ['b', 3]]

What should be the right list comprehension and why is my list comprehension not working?

Answer 1

b = [[i, j] for i in a for j in a[i]]

for nested for loops in list comprehension the first loop will be the one whose variable you will be using in the second one, like here for example i is used in the second loop, nested for loops in list comprehensions are hard to read therefore you should better avoid it.