Extracting list elements from dictionary of lists in python using list comprehension

Question

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

and I need

[1,2,3,4,5] 

using list comprehension. How do I do it?

Answer 1

Use a nested list comprehension:

>>> [val for lst in d.values() for val in lst]
[1, 2, 3, 4, 5]

But you may need to sort the dictionary first (because dicts are unordered) to guarantee the order:

>>> [val for key in sorted(d) for val in d[key]]
[1, 2, 3, 4, 5]

Answer 2

Easy and one liner solution using list comprehension :

>>> sorted([num for val in d.values() for num in val])
    [1, 2, 3, 4, 5]

Answer 3

sum(d.values(),[]) works but not performant because it applies a = a + b for each temp list.

Use itertools.chain.from_iterable instead:

import itertools
print(list(itertools.chain.from_iterable(d.values())))

or sorted version:

print(sorted(itertools.chain.from_iterable(d.values())))

Answer 4

If the order of elements is not important, then you cannot beat this:

sum(d.values(), [])

(Add sorted() if necessary.)