How to change the positions of a dictionary?

Question

I have a dictionary as follows:

d = {'a': ['b'], 'c': ['d']}

I want to change the positions of the dictionary so that it looks like the following:

d_new = {'b': 'a', 'd': 'c'}

I have tried the following, but due to the second term being a list in my original dictionary (d), I am unable to complete this.

d = {'a': ['b'], 'c': ['d']}

for k in list(d.keys()):

    d[d.pop(k)] = k

    print(d)

The operation you're wanting to do is customarily called inverting or reversing a dictionary — i.e. swapping its keys and values. — martineau, Apr 26 '22 at 08:26

score 2 · Answer 1 · answered Apr 26 '22 at 06:26

You can use iterable unpacking in a dict comprehension like so:

{v: k for k, (v,) in d.items()}

>>> d = {'a': ['b'], 'c': ['d']}
>>> {v: k for k, (v,) in d.items()}
{'b': 'a', 'd': 'c'}

This assumes all values are a list of one element and you only want the first element in the lists.

Otherwise you can use this more appropriate code:

{v[0] if isinstance(v, list) else v: k for k, v in d.items()}

score 0 · Accepted Answer · answered Apr 26 '22 at 06:37

A variation on a theme using list unpacking:

d = {'a': ['b'], 'c': ['d']}

d = {v: k for k, [v] in d.items()}

print(d)

In case the values (lists) happen to contain more than one element and you're only interested in the first element then:

d = {v:k for k, [v,*_] in d.items()}

Of course, this could also be used even if there's only one element per list

2 Answers2